Abstract: Tomatoes are commonly cultivated in solar greenhouses worldwide to extend the growing seasons for high yields. However, the air circulation has been severely restricted by the high airtightness of the solar greenhouse structures at the low nocturnal temperatures of the cold seasons in winter. This low airflow has also led to the accumulation of the water vapor released by the tomato plant transpiration and soil evaporation. Thereby, the high air relative humidity can last for the extended nightly periods. Such high-humidity environments can significantly increase the susceptibility to Botrytis cinerea infection—a destructive fungal disease that impairs tomato quality to reduce the marketable yields. In the present study, the refrigerated dehumidifiers were introduced into the nocturnal environment of the solar greenhouses. Among them, one unit was placed at the northeast corner, and another was at the southwest corner of the experimental greenhouse, particularly for the uniform dehumidification coverage. The high-precision sensors were deployed at the representative sampling points in both the test area (with dehumidifiers) and the control area (without dehumidifiers). Air temperature and relative humidity were measured after deployment. A systematic comparison was conducted on the dynamic differences in the temperature and humidity parameters in the two areas. A systematic investigation was implemented to explore the regulation of the condensation dehumidification on the nocturnal humidity environment of the solar greenhouses. The preventive efficacy was quantitatively evaluated against Botrytis cinerea. The results demonstrated that the refrigerated dehumidifiers were operated effectively under the typical "low-temperature and high-humidity" nocturnal conditions in the solar greenhouses, with a stable operation temperature threshold of 15 °C for the ambient air. The equipment ran steadily under suitable temperature conditions above15℃.The average temperature difference (ΔT) between the inlet and outlet of the dehumidifier reached 13.5℃, the average relative humidity difference (ΔR_RH) was 44.8%, the effective dehumidification duration approached 100%, the dehumidification capacity (D) was 1.31 kg/h, and the dehumidification energy consumption index (R) was 0.77 kW·h/kg, indicating the economical energy efficiency. In contrast, the refrigerated dehumidifiers were operated intermittently with the periodic fluctuations under unsuitable temperature conditions below 15℃, due to the necessary defrosting. The defrosting cycles also interrupted the continuous dehumidification, thereby weakening the equipment’s temperature-raising and humidity-reducing. At the same time, the average ΔT and ΔR_RH between the inlet and outlet were reduced to 7.6℃ and 29.5%, respectively, while the D decreased to 0.99 kg/h and R increased to 1.01 kW·h/kg. The dehumidification performance of the equipment was significantly influenced by the ambient air temperature at nighttime. Condensation dehumidification effectively regulated the nocturnal humidity environment in the solar greenhouses. The nocturnal relative humidity was maintained at approximately 80%, which was 14.2% lower than that of the control plot, while the nocturnal air relative humidity was reduced by 6.7-10.6 percentage points. Additionally, there was an increase in the nocturnal saturation vapor pressure deficit to 0.4 kPa—a parameter closely related to inhibiting fungal spore germination—that exerted negligible impacts on the air temperature and lowered the dew point temperature by 0.4-2.8℃. Notably, the low-temperature air failed into the greenhouse during dehumidification, thus preventing any adverse reduction in the indoor air temperature for the favorable conditions of the tomato growth. Condensation dehumidification also showed a remarkable ecological preventive effect on the tomato leaf Botrytis cinerea. The duration was reduced and the frequency of the high relative humidity above 85% periods at nighttime. The initial infection was effectively inhibited to avoid the next spread of the Botrytis cinerea. Statistical analysis showed that the dehumidification treatment was achieved in a preventive rate of 85.04% for the disease index and 73.74% for the disease incidence rate of the tomato leaf Botrytis cinerea, thus outperforming many conventional physical control measures. Furthermore, the optimal intermittent mode of the dehumidifiers can be expected to reduce the operational costs. While the stable dehumidification efficacy was maintained by adjusting the operation intervals, according to the real-time humidity monitoring data. In conclusion, the condensation dehumidification can effectively improve the high-humidity nocturnal environment in the solar greenhouses and then prevent the occurrence of the tomato leaf Botrytis cinerea. This finding can provide a possible technical solution for the precise regulation of the nocturnal humidity environments and the green prevention and control of diseases in solar greenhouse tomato production. The practical significance can also be used to promote the sustainable protected horticulture.