Abstract: 【OBJECTIVE】In order to accurately reflect the photosynthetic characteristics and requirements to environmental conditions of sweet potato trees under hydroponics, the characteristics of photosynthesis of Yanshu18 and Yanshu20 cultivars were studied comparatively with the CIRAS-1 photosynthetic instrument. The main results were as follows: diurnal change in net photosynthetic rate (Pn) of two varieties of sweet potato trees were a typical single curve determinately regulated, the responsive curves of photosynthesis in mature tomato leaves to the change of photosynthetic active radiation, CO2 concentration and air temperature could be described with quadratic equations. The light saturation point of Yanshu18 and Yanshu20 were around 1704.721-1925.037 μmol/(m2?s), the light compensation point was about 33.735-39.857 μmol/(m2?s), the proper temperature was around 25℃, CO2 saturation were 776.406-819.342 μL/L, and CO2 compensation were 55.667-62.410 μL/L.