Elymus nutans is the prior crop to establish the pasture or improve the pasture in the alpine regions because it is cold-resistant tolerance with good quality. Although proper plant density and nitrogen application rate are conserved as the key factors to affect the yield stability of Elymus nutans pasture, whether there is an optimally theoretical combination of plant density and nitrogen application rate for maintaining the relative yield of Elymus nutans is not well documented. A pot experiment was conducted in a greenhouse to investigate the effect of different combination of plant densities and nitrogen application rates on the plant height, tiller per plant, aboveground biomass, underground biomass, root volume and aboveground to underground biomass ratio, which can be used to identify an optimally theoretical combination between plant density and nitrogen application rate. In this study, the plant densities consisted of 58, 102, 146 plants per square meter and nitrogen application rate was 0, 200, 400 mg/kg. This study showed that with the increase of plant density, the plant height, aboveground biomass and aboveground to underground biomass ratio of Elymus nutans increased first and then decreased, while the tiller per plant decreased gradually, the root volume and underground biomass increased first and then remained relatively stable. The tiller per plant, aboveground biomass and aboveground to underground biomass ratio of Elymus nutans increased first and then decreased with the increase of nitrogen application rates, and the underground biomass decreased gradually. Interaction between plant densities and nitrogen application rates was found to had no effect on root volume and tiller per plant, whereas it was observed to have significant effects on the plant height, aboveground biomass, underground biomass, and the aboveground to underground biomass ratio. The relationships between above-mentioned parameters (plant height, aboveground biomass, underground biomass, and the aboveground to underground biomass ratio) and nitrogen application rate and plant density showed the downward 3D response surfaces, and this demonstrated that there was an optimally theoretical combination between proper plant density and nitrogen application rate, under which plant height, aboveground biomass, underground biomass, and the aboveground to underground biomass ratio were the best. In this study, the optimal combination of plant density and nitrogen application rate were 102 plants per square meter and the nitrogen application rate was 200 mg/kg. The findings of this study suggest that there is an optimally theoretical combination of plant density and nitrogen application rate for maintaining the relative yield of Elymus nutans pasture, which can provide a scientific basis for high persistence yield and optimal allocation of above/underground biomass of Elymus nutans pasture in the field management.