The construction of artificial grassland is the necessary way to alleviate pressures on the grassland in Inner Mongolia. Water shortage is the major limiting factor for grass growth in artificial grassland in this region. Suitable irrigation quantity is the key to solve the problem of water shortage by both increasing grass yield and saving water. In this study, we selected two local typical grasses Agropyron cristatum and Medicago sativaL and conducted irrigation experiments with single sowing and mixture sowing of the two. Then comparative analyses were explored on biomass, photosynthesis and water use efficiency of the two grasses under different irrigation schemes. The results show that: (1) The mixture sowing of leguminous herbage and gramineous herbage is helpful to enhance the yields of the two plants.; (2) Squaring period in the early August is the best season for the harvesting of Agropyron cristatum and Medicago sativaL, when the yields of the grasses are the highest; (3) Treatment 3, of which the irrigation quantity is up to 45% of the field capacity, the amount of water recharge per unit (including precipitation and irrigation) is 903.8 m³/hm² in early July and 1812.4 m³/hm² in early August, is the best water treatment choice for both grass yields increase and water-saving; (4) Rain-fed condition or irrigating a little water (treatment 3) is effective to increase long-term water use efficiency (WUEL) of the grasses from June to August. Irrigating more water (treatments 1 and 2) started to affect WUEL only after entering the flowering stage of the grasses; (5) Rain-fed condition or irrigating a little water is effective to increase grass leaf instantaneous water use efficiency (WUEI) in early August. However, because of intensive sunlight, water evaporation is high and it is more effective to increase WUEI by irrigating more water in early July.