Abstract: A de Wit replace series experiment was used to explore the competitive relationship of two oat varieties of Dingyan 1 and Jinyan 17 under different levels of water and fertilizer application. The results showed that Jinyan 17 had greater competitiveness than Dingyan 1 under a series of water and fertilization conditions. Under low water +  fertilization or sufficient water + fertilization, the aboveground biomass of Jinyan 17 was significantly higher than that of Dingyan 1. With the increasing levels of water and fertilization, the ratio of stem biomass to aboveground biomass of Dingyan 1 and Jinyan 17 increased, and the ratio of leaf biomass to aboveground biomass of both oat varieties decreased. In all treatments, grain yield in monoculture and harvest index of Jinyan 17 were greater than those of Dingyan 1. By simultaneous increase of aboveground biomass and harvest index, Jinyan 17 obtained higher grain yield. The synchronous improvement of competitive ability and harvest index is expected to become the focus of crop breeding in arid areas.