The photosynthetic characteristics and water use of WL323MF at 3 irrigation treatments (60㎜, 40㎜ and 20㎜ water were applied at the regeneration period and after mowing, respectively) were studied in Beijing's farmland. The results showed t㏊t there are the diurnal change with "M" shape in the Pn, Tr and Gs of WL323MF and there are the positive corresponding relationship between the ㏊y yield and the water consumption, the WUE. About 60㎜ of irrigation could get about 532.74㎜ of the water consumption and 17925.28㎏/㏊ of alfalfa hay, about 20㎜ only could get about 389.60㎜ of the water consumption and 10450.75㎏/㏊ of alfalfa hay.