With 2 years alfalfa (Medicago sativa) grown under apple trees as material,0.150,0.225 （control） and 0.300 m ³/m ²,three water amount levels were set up.The effects of different irrigation grads on biomass,photosynthetic parameters,chlorophyll content,proline content and relative electric conductivity of alfalfa were studied.Our results showed that:(1)Soil moisture content and above-ground biomass of alfalfa was significantly affected by the irrigation,and it increased with the increasing irrigation amount.Under high irrigation amount treatment the roots tended to distribute in surface layer.(2)Net photosynthetic rate (P_n) of alfalfa increased with the increase of photosynthetic active radiation (PAR) under all irrigation treatments.It increased rapidly under lower PAR value,and ascended smoothly when PAR reached certain value.P_n increased with increase of irrigation amount.(3)Chlorophyll content in leaves of alfalfa increased after irrigation.Relative electric conductivity in leaves of alfalfa decreased with the increase of irrigation amount.Proline accumulated significantly under lower irrigation amount,indicating that it may play an important role in drought resistant.Irrigation can effectively improve the soil water condition and increase photosynthetic productivity of alfalfa.