According to the complete random split spot design,the effects of different irrigation water amount and phosphorous supply on the water srtorage,water consumption rate,water utilization efficiency(WUE) of soil under alfalfa,and root weight of alfalfa were analysed taking irrigation water(330mm of normal irrigation, 164mm of 20%saved irrigation and 198mm of 40%saved irrigation)as the main factors and phosphorus supply(0,60,120and 180kg/hm²)as the cofactor.It was indicated that the soil water storage,the water consuming capacity and root weight of alfalfa increased with the irrigation water increasing while the water utilization efficiency decreased with it.Phosphorus supply did not significantly affect soil water storage and water consumption rate.The WUE and dry root weights of alfalfa at 0-40cm depth increased with P supplies'increasing at first,and decreased later while the WUE and the whole life dry root weight reached its maximum value at 26.50kg/(mm·hm²)and 1 320.78g/m² when the phosphorous application rate was 120kg/hm².