Abstract: Four irrigation treatments (0,25,50,75 mm per time, 3 times in a year round) were designed to study the productivity response of alfalfa planted 3 years ago in south of Beijing through measuring the yield, leaf to stem ratio, plant height in early flowering stage and dry matter of single plant of second and third harvests. The results showed that if enough water was supplied in last winter, the irrigation treatment did not affect the yield of the first harvest, but the second harvest, third harvest and annual yield increased with the irrigation amount. However, the yield of fourth harvest was reduced. Plant height in all harvesting times showed the same trend with the yield. In addition, the irrigation did not significantly affect the leafstem ratio of the first and fourth harvests. Stemleaf ratio and weight of single branch in the second and third harvesting times increased with the irrigation amount. Therefore, irrigation should be performed after the first and second harvests. it should not be irrigated at the growing period after third harvest and the stage of turning green if irrigated in last winter. The best annual irrigation amount was 150 mm while the rainfall in growing season was 390 mm.