In order to promote the planting of Xanthoceras sorbifolia Bunge in Semiarid area, this study took 3yearold seedlings of X. sorbifolia Bunge as materials in Loess Hilly Region in which its gas exchange and diurnal variation of chlorophyll fluorescence parameters under different moisture conditions were measured and the quantitative relationship between photosynthesis and different soil moisture was analyzed. The results indicated that:(1) with the decrease of relative soil water content, the net photosynthetic(P_n),water use efficiency(WUE),maximal fluorescence under light(F_m), PSⅡ maximum light quantum yield (F_v/F_m), PSⅡ practical quantum efficiency(Φ_PSⅡ)and photochemical quenching(qP) increased at first and then decreased, minimum fluorescence (F_o) decreased first and then increased, and NPQ continued to rise. All parameters were maintained at a higher level when W_r was 44.7%-81.2%. (2) The daily net photosynthetic accumulation and water use efficiency (WUE) were at a higher level while W_r was at 58.6%-81.2%. WUE reached the maximum value when W_r was 66.6%. It is not favorable to the photosynthesis and water utilization of X. sorbifolia Bunge when the soil water content was too high or too low. (3) Gas exchange data showed that the decline of photosynthesis at noon was caused by stomatal factor when W_r was at 38.1%-81.2%, but it was restricted to nonstomatal factor when W_r was less than 31.8%. There would be photo inhibition at noon and it would damage the photosynthetic mechanism while W_r was less than 73.9%. In short, it is suitable for the cultivation of X. sorbifolia Bunge in Loess Hilly Region while W_r was at 58.6%-81.2% and it cannot grow normally while W_r was less than 38.1%.