A study was conducted to determine the fitting soil moisture for the normal growth of two-year-old W. sinensis (Sims) Sweets by using gas exchange technique. Remarkable threshold values of net photosynthetic rate (Pn) transpiration rate (Tr) and water use efficiency (WUE) were observed in the W. sinensis leaves treated by various soil moisture and photosynthetic available radiation (PAR). The fitting soil moisture for maintaining a high level of Pn and WUE was in range of 15.3%–26.5% of volumetric water content (VWC) of which the optimal VWC was 23.3%. Under the condition of fitting soil moisture the light saturation point of leaves occurred at above 800µmol·m-2·s-1 whereas under the condition of water deficiency (VWC 11.9% and 8.2%) or oversaturation (VWC 26.5%) the light saturation point was below 400µmol·m-2·s-1. Moreover the light response curves suggested that a special point of PAR occurred with the increase in PAR. This special point was considered as the turning point that indicated the functional transition from stomatal limitation to non-stomatal limitation. The turning point was about 600 1000 1000 and 400 µmol·m-2·s-1 respectively at VWC of 28.4% 15.3% 11.9% and 8.2%. In conclusion W. sinensis had higher adaptive ability to water stress by regulating itself physiological function.