Abstract: OBJECTIVE To study the relationship between photosynthetic characteristics and environmental factors in leaves of Alisma orientalis. METHODS The daily variation of net photosynthetic rate， environmental factors and photo-response in leaves of A. orientalis were measured by LI-6400 Portable Photosynthesis Apparatus. RESULTS The net photosynthetic rate (Pn) and stomatal conductance (Gs) showed a diurnal variation of bimodal curve. The transpiration rate (Tr)， intercellular CO2 concentration (Ci) and stomatal limitation (Ls) presented a single peak diurnal variation. The water use efficiency (WUE) was a single valley type of diurnal variation. Correlation analysis indicated that there were positive correlations between Pn of A. orientalis and photosynthetic active radiation (PAR)， Gs， Tr or WUE， but negative correlations between Pn and relative air humidity (RH)， Ci or Ls. CONCLUSION A. orientalis is the type of stomatal limitation and sun plant. The net photosynthetic rate of A. orientalis leaves has an obvious "midday depression" phenomenon. The results suggest that stomatal conductance， water use efficiency， photosynthetic active radiation and transpiration rate are the main environmental factors influencing the A. orientalis photosynthetic rate. This study could offer certain theoretical reference for A. orientalis rational cultivation， as well as provide a scientific basis for improving the quality and production of medicinal materials.