Rapeseed is mainly cultivated in hilly areas in China and the inclination angle is an important parameter for influencing the seeding quality of a seed-metering device. However, the relationship between seeding performance and inclination angle remains unclear. This study analyzes the influence of inclination angle (θ) on the characteristics of seed-filling and seeding performance in a centralized seed-metering system of rapeseed using DEM simulation and the bench test. The results revealed that the inclination angle (θ) had a significant effect on seed-filling performance. The missing seeds ratio and the deviation of seed layer height increased with an increase in a lateral inclination angle. The missing seeds ratio in the forward direction (−Y direction) and seed-filling angle, double seed-filling ratio, and seed layer height in the back direction (Y direction) increased with the increasing inclination angle. Under seed-cleaning treatment, the missing seeds ratio was significantly reduced in the −Y direction at a seed layer height of 25 mm, and the qualified ratio was more than 91.0% at a seed layer height of 20 mm in X direction. The phenomenon of seeds passing through the seed-metering wheel disappeared with a qualified ratio larger than 90.0% in Y direction by installing a seed-cleaning brush. The results contributed to improving the knowledge of seed-filling characteristics of the centralized seed-metering device at inclination conditions and should help to improve seeding performance by adjusting the seed layer height and installing a seed-cleaning brush.