Abstract: Stable and appropriate temperature is the key factor to ensure the normal operation of biogas projects in cold regions during winter. To clarify the temperature variation patterns inside biogas fermentation tank in cold regions, reasonably determine the heating load of temperature regulation systems, provide basis for their optimized design, and ensure the efficient, economical, and stable operation of biogas projects, this study established a thermal environment model for solar greenhouses integrated with biogas fermentation tank and applied it to practical engineering cases. This study theoretically analyzes the coupled thermal balance relationship between the biogas solar greenhouse and the fermentation tank, establishes the thermal environment model of biogas solar greenhouse and fermentation tank based on the law of energy conservation, and derives a calculation formula for the heating load of the fermentation tank. Using the COMSOL Multiphysics numerical simulation method, the temperature variation patterns inside the solar greenhouse and biogas fermentation tank were determined. The results were validated through field experiments and further applied in a practical engineering case in Xuzhou. The numerical simulation of temperature inside the solar greenhouse, soil, and fermentation tank shows good agreement with the on-site experimental test results, with maximum relative errors of 9.80%, 7.51% and 2.38% respectively, and average relative errors of 3.73%, 3.09% and 1.27%, meeting engineering accuracy requirements and confirming the feasibility of the established mathematical model and numerical simulation approach. Both field measurements and simulation results demonstrated that: the mean air temperature in the greenhouse, mean soil temperatures and fermentation tank temperature all exhibited irregular periodic variations, with their peak times showing certain lag compared to outdoor temperature peaks. During the testing period, the maximum greenhouse air temperature reached 49.6℃, significantly higher than the maximum outdoor temperature of 15.7℃, indicating the greenhouse's excellent heating performance. Taking a practical project in Xuzhou as an example, the thermal environment model was applied in engineering practice. Numerical simulations were conducted to study temperature variation patterns of the solar greenhouse and fermentation tank under typical meteorological year conditions, as well as the fermentation tank hourly heating load. Results revealed the temperature variations in both greenhouse and fermentation tank, and the changes of fermentation tank hourly heating load in response to outdoor environmental conditions. The greenhouse air temperature was significantly influenced by outdoor environment. During daytime when outdoor temperatures were higher and solar radiation intensity was greater, correspondingly higher temperatures were achieved inside the greenhouse, while nighttime temperatures were relatively lower. The annual temperature variation range was 10.02-64.86 ℃, with the maximum value occurring at the 4 550th hour, lagging 166 hours behind the outdoor temperature peak at the 4 384th hour. Throughout the year, the fermentation tank temperature varies from 26.51 to 37.63 ℃ with 3 451 hours falling below the optimal mesophilic fermentation range (30-45 ℃) , accounting for 39.39% of total annual time. The fermentation tank hourly heating load showed substantial variations (409.93-3.62×10⁵ kJ/h) in response to outdoor environmental changes. The heating system design should be based on maximum heating load requirements, while operational flexibility allows for energy-saving adjustments according to gas demand. The thermal environment model of the biogas solar greenhouse and fermentation tank established in this study demonstrates high accuracy and reliability, providing robust theoretical and technical guidance for optimizing the heating and insulation design of biogas engineering systems.