Aims To maximize the productivity of oats, and evaluate the effects of sowing density and fertilizer rate on the yield, root growth, leaf area index (LAI), water and fertilizer use efficiency of rainfed oats. Methods In 2022 and 2023, a two-factor randomized block design field trial was conducted under no irrigation in Northwest China during the oat planting season. Three sowing densities (low density (L): 75 kg/ha; moderate density (M): 150 kg/ha; high density (H): 225 kg/ha) were coupled with six fertilizer rates (N0P0: no fertilization; N2P0: 100 N kg/ha; N0P2: 90 P kg/ha; N1P2: 50 N kg/ha, 90 P kg/ha; N2P1: 100 N kg/ha, 45 P kg/ha; N2P2: 100 N kg/ha, 90 P kg/ha). Results The aboveground dry matter yield, grain yield, root length density, root surface area density, root biomass, leaf area index (LAI), water use efficiency (WUE) and partial N fertilizer productivity (NFP) increased by 9.9-204.1%, 4.2-90.5%, 9-48.8%, 15.8-45%, 18.6-24.5%, 14.9-78.9%, 17.8-43.8% and 15.8-248.3% respectively, under M-N1P2 compare with other treatments. The results revealed significant positive correlations (P < 0.05) of DM and grain yield with root system, LAI, WUE and NFP. M-N1P2 increased the WUE and NFP by improving root structure and LAI, resulting in the highest DM yield and grain yield. Conclusion The sowing density of 150 kg ha(-1) coupled with 50 kg ha(-1) N and 90 kg ha(-1) P is recommended in the semi-arid regions to promote oats production.