Nitrogen fertilizer application and planting densities were key limiting factor for the physiological metabolism and growth of Avena sativa L. In the paper, it was set five nitrogen fertilizer application (including: N1:225 kg/hm2, N2:300 kg/hm2, N3:375 kg/hm2, N4:450 kg/hm2, N5:525 kg/hm2) and four planting densities (including M1:3 million plants/hm2, M2:3.5 million plants/hm2, M3:4 million plants/hm2, M4:4.5 million plants/hm2), and measure plant height, aboveground biomass, underground biomass, total biomass, stem biomass, leaf biomass, root biomass and ear biomass, and calculate stem/leaf ratio, root/crown ratio, stem contribution rate, leaf contribution rate, root contribution rate and spike contribution rate, to study the effect of density and nitrogen fertilizer on height, biomass and material distribution of Avena sativa L to provide scientific basis for water and fertilizer management of Avena sativa L in Horqin Sandy Land (Science and Technology Park of Inner Mongolia university for nationalities). The results showed, The main purpose was to harvest aboveground biomass such as oat stem and leaf, the optimal density and nitrogen fertilizer combination was M4 and N3, and aboveground biomass was 4254.96 g/m2 (2017) and 4226.21 g/m2 (2018). Under the same planting density, the height, aboveground biomass and underground biomass of Avena sativa L all showed first increased and then decreased with the rising of fertilizer application and the biggest height value appeared at N3. Under the same nitrogen fertilizer application, the height indicated increased gradually with the rising of density, and higher biomass was obtained under lower nitrogen fertilizer application and higher planting density conditions (such as:N1 conditions, the biggest value of underground biomass and total biomass was obtained under M1 condition), and under higher nitrogen fertilizer application conditions the higher value of biomass was obtained under lower planting density condition (M1 (2017) and M2 (2018) under the condition of N5 treatment, the aboveground biomass and total biomass were the highest, while M4 (2017) and M3 (2018) underground biomass was the highest.). Under same nitrogen fertilizer application conditions, the root/shoot ratio and stem/leaf ratio all showed increased first and then decreased with the rising of density. Under same density conditions, the root/shoot ratio and stem/leaf ratio indicated different variation laws with different nitrogen fertilizer application amount. The changes laws of root contribution rate, stem contribution rate, leaf contribution rate and ear contribution rate were different due to density and nitrogen fertilizer application.