Abstract: This study is based on the two years irrigation experimental data of grasses, Clinelymus Sibiricus(L.), Clinelymus Dahuricus, Agropyron Mongolium Keng and Medicago Sativa L., perennial cultivated forage grass to simulate each grass evapotranspiration(ET) by using dual crop coefficient approach presented in FAO-56. The relationships between grass water requirement and grass hay yield, between grass water requirement and water use efficiency and between grass water requirement and marginal product were analyzed The high efficient irrigation schedule with different probabilities of effective rainfall was determined under the objective of maximizing water use efficiency.