Abstract: Abstract: The growth and physiological metabolism of plant are greatly affected by root-zone temperature. The heat stress around root zone in hot summer is an important factor limiting the seedling growth. Compared with air temperature cooling, root-zone cooling is more economical and is an effective solution to alleviate high temperature stress. A new water-cooled seedling bed (WSD) has been proved to be an effective method to lower the root-zone temperature. In order to utilize WSD scientifically, 5 different root-zone cooling periods were designed using the WSD device for root-zone cooling of tomato (Solanum lycopersicum) seedling. The 5 different root-zone cooling periods were as follows: M1 (cooling from 7:00 to 11:00), M2 (cooling from 11:00 to 15:00), M3 (cooling from 7:00 to 19:00), M4 (cooling from 19:00 to 7:00) and M5 (cooling all day). Non-cooling seedling bed was used as control. The objective of this study was to investigate the cooling effect of the WSD under 5 different root-zone cooling periods and its effect on the growth of tomato seedling. The root-zone temperature, seedling growth, root morphological traits and physiological characteristics of tomato seedling were tested. The results showed as follows: The root-zone daily mean temperature and daily maximum temperature of tomato seedlings under 5 cooling periods of the WSD were all lower than those under the non-cooled seedling bed (CK) during tomato seedling period. The cooling effect of the WSD was significantly (P<0.05) correlated with cooling duration and cooling period. Compared with cooling during night, mean temperature in the daytime, daily mean temperature and maximum temperature for the treatments cooling during the day time were lowered significantly (P<0.05). Compared with other treatments, the treatment M3 (cooling from 7:00 to 19:00) and M5 (cooling all day) got the best cooling effects. The daily mean root-zone temperatures of tomato seedlings under M3 and M5 were 23.6 and 22.6℃ respectively, which were lowered by 3.3 and 4.3℃ respectively compared with CK during tomato seedling period. The daily variation of root-zone temperature of tomato seedlings was affected also by cooling duration and cooling period. Root-zone temperature of treatments cooling during the day time showed a small variation, while root-zone temperature of treatments cooling during the night got a big fluctuation. The daily variation of root-zone temperature under M4 was maximum among all treatments and its diurnal temperature range reached 10.3℃. In contrary, the daily variation of root-zone temperature under M3 was minimum among all treatments and its diurnal temperature range was only 1.7℃. Dry mass and leaf area per plant of tomato seedling in 5 treatments were enhanced significantly, of which the differences of the 2 growth indices between M3, M5 and CK reached significant level (P<0.05). Tomato seedling stem diameter and healthy index under the M3 treatment were improved by 35.1% and 39.5% respectively compared with the CK treatment. Total length of root, toot surface area, chlorophyll content and root activity were all improved in the 5 treatments. Considering the factors such as energy conservation and strong seedling culturing, root zone cooling from 7:00 to 19:00 (M3) is an effective way for the scientific utilization of water-cooled seedling bed.