Siberian wildrye grass (Elymus sibiricus L.) is the primary forage species in alpine cold areas of North China. Scheduled irrigation is an important way to increase the forage mass because of water shortage and the inconsistency between rainy season and the most intense water use period. Irrigation experiments were conducted during 2006 to 2008 to study the feasibility of applying single irrigation before winter (WI) or at the elongating stage (EI), irrigating at these two stages (WEI, in 2006–2007), and irrigating before winter with mulching straw before next reviving (WMI, only in 2008) to increase forage mass (FM) and water use efficiency (WUE). The results showed that irrigation at the elongating stage played the most important role in mass and WUE increase. The FMs under EI achieved 6381, 6883, and 5763 kg ha−1, increased by 66, 218, and 99%, respectively, and WUE was 1.9, 2.3, and 1.8 kg m−3, increased by 56, 134, and 81%, respectively, compared with no irrigation treatment (NI) in 3 yr. The FMs under WI were increased by 40, 53, and 44%, and the WUE was increased by 30, 31, and 34%, respectively. No significant FM and WUE increment under WEI were found compared with EI during 2006 to 2007. The FM and WUE under WMI were higher than values under EI in 2008. Therefore, EI can achieve relatively high production and WUE of Siberian wildrye grass. If water resources at the elongating stage were deficient, the WMI treatment might be a good choice.