We aimed to investigate water response by assessing physiological photosynthetic parameters of Salix matsudana growing on the Shell Ridge Islands,and to determine their moisture adaptability in a sandy habitat formed from seashells.We measured net photosynthetic rate,light response,chlorophyll content,leaf water potential and other parameters of S.matsudana grown in soils with different moisture contents,simulating the natural moisture gradient of the shell sand habitat.We found that:(1)A rectangular hyperbolic correction model best simulates the light response of S.matsudana leaves (R²>0.96).High photosynthetic capacity is maintained at 50.1%～94.4% soil relative water content (RWC) under optimal light intensities ranging from 800～1 600 μmol·m^-2·s^-1.(2)Photosynthetic light response of S.matsudana leaves show a certain threshold response to soil moisture content.Net photosynthetic rate,light saturation point,apparent quantum yield (AQY) and the maximum net photosynthetic rate all demonstrate optimal capacity at medium soil moisture content.Soil moisture content can significantly affect the light use efficiency and the light ecological amplitude of the plant.Under drought or waterlogged stress conditions,the light compensation point of S.matsudana increases,while its light saturation point decreases,light ecological amplitude narrows and light use efficiency decreases.(3)Instantaneous water use efficiency and potential water use efficiency of S.matsudana leaves reach their maximum values when RWC is 45.7% and 40.6%,respectively.Thus,we conclude that moderate drought stress can significantly increase water use efficiency of S.matsudana growing in a shell sand habitat.(4)Both drought and waterlog stress can significantly lower chlorophyll content and leaf water potential.The chlorophyll content and leaf water potential reach their maximum values when RWC is 58.9% and 50.1%,respectively.Our study shows that net photosynthetic rate,photosynthetic light response parameters,water use efficiency,chlorophyll content and leaf water potential of S.matsudana living in a shell sand habitat have distinct threshold responses correlated with soil moisture.In conclusion,S.matsudana is found to be a species with wide water ecological amplitude,which is resistant to high soil moisture content and susceptible to drought.