Abstract: Abstract: As the main plant organ of absorbing and transporting water and nutrients, root system and its metabolism directly affect plant growth and development．Plant growth is more susceptible to root zone temperature than above ground portion. The root zone temperature greatly affects the growth and physiological metabolism of plant. Optimal and stable rhizosphere temperature is an important factor for root growth and metabolism. The heat stress around rhizosphere during summer season is an important factor limiting the seedling growth. Root-zone cooling is more economical compared with air temperature cooling, it can be an effective solution to alleviate high temperature stress. A new water-cooled seedling bed (WSD) was exploited by using the underground water as the natural coolant to lower the root-zone temperature during summer season. The root cooling system consisted of inlet pipe, cooling tube, return pipe and control valve. The profile of cooling tube with a trapezoidal cross section was used, which could increase the contact area between cooling tube and plug trays. Tomato seedlings were grown in 72-cell plug trays that were cooled by cold water flowing through the cooling tube buried at the bottom of plug trays. The objective of this study was to investigate the cooling effect of water-cooled seedling bed and its effect on the growth of tomato seedling. The root zone temperature, growth and physiological characteristics of tomato seedling were analyzed in this study. The results showed that the cumulative temperature, mean daily temperature and mean daily maximum temperature of the root-zone in WSD were on average, 154.1, 4.5 and 6.5℃ lower than those of non-cooled seedling bed (NSD) during tomato seedling period, respectively. The mean temperature diurnal variation of WSD (8.5℃) was milder than that of NSD (13.6℃). The root-zone temperature was analyzed. The results showed that the mean high temperature duration of WSD above 25 and 28℃were 8.0 and 3.1 h per day, respectively. However, the mean high temperature duration of NSD reached 15.6 and 10.3 h. The run length of WSD was shortened by 7.6 and 7.2 h at the fixed level 25 and 28℃, respectively. The leaf evaporating rate of tomato seedling in cooled treatment was enhanced by 36.3% compared with that of non-cooled treatment, which improved the temperature difference between leaf and air. Root activity and the net photosynthetic rate of tomato seedling were also improved significantly. Plant height, dry mass of shoot and root of tomato seedling in cooled treatment were increased by 35.8% and 72.6% compared with seedling of non-cooled treatment, while no significant difference was observed in the stem diameter of seedling between two treatments. Tomato seedling healthy index in the water-cooled seedling bed was improved by 57.1% compared with a non-cooled treatment. This study indicated that the water-cooled seedling bed is an effective method to alleviate heat stress around rhizosphere and improve the seedling growth during summer season.