Abstract: To optimized the water and fertilizer application for alfalfa cultivation, growth and hay yield of alfalfa were studied in the semiarid region of northern Shanxi province. Four treatments of irrigation (W0 with no irrigation, W1 with irrigation of 75mm once, W2 with irrigation of 75 mm twice, W3 with irrigation of 75 mm three times) and three treatments of phosphorus fertilizer application (P0 with 0 kg P2O5/hm2, P1 with 105 kg P2O5/hm2, P2 with 210 kg P2O5/hm2) were conducted by the split-plot design. Phosphorus fertilizer improved the winter hardiness of alfalfa, which exhibited delaying brown in autumn and earlier green in spring. Later flowering and less flowers of alfalfa for the second growth period in 2004 were shown with the increasing of irrigation times and phosphorus fertilizer application. The hay yield of alfalfa for the first harvest, the second harvest and the whole year in 2004 and 2005 were significantly increased with the irrigation times and phosphorus fertilizer application. The effects of irrigation or phosphorus fertilizer on alfalfa production were extremely significant, while the interactive effect of irrigation and phosphorus fertilizer was not significant. Two times irrigation (W2) for the first harvest and the second harvest, respectively, no irrigation (W0) and low phosphorus fertilizer application (P1) for the third harvest were recommended for alfalfa cultivation.