Artificial rainfall experiments were carried out to investigate the distribution characteristics of nitrogen and phosphorus in runoff system of urban backfill soils of different slope gradients. The results showed that soil slope had a significant effect on hydraulic distribution. Specifically, when the slope was 4°, the proportion of surface runoff, side-percolation flow, and down-percolation flow was 72.5%, 25.3%, and 2.3%, respectively. However, the surface runoff was prominent when the slope reaches 8°. Furthermore, for the slope of 4°, the percentage of total nitrogen in the surface runoff, side-percolation flow, and down-percolation flow was 69.7%, 26.1%, and 4.2%, respectively. Meanwhile, the proportion of total phosphorus was 77.6%, 22%, and 0.4%, respectively. The proportion of nutrients in surface runoff increased slightly with increasing slop gradient. Therefore, in the design of backfill soil, smoothing the surface would be beneficial to improve soil water infiltration, reduce surface runoff, and decrease nitrogen and phosphorus pollution of urban backfill soil.