An air-assisted centralized metering device for rapeseed was designed based on diffusion ring, including seed supply device, seed transport pipe, diffusion ring, distributor, seed discharge pipe, air pump, seed box, etc. The top angle of the distributor and the diffusion ring configuration were optimized for the bowl-type branch distributor. The influence of top angle of the distributor and diffusion ring configuration on the seed uniformity of the rapeseed air-assisted centralized metering device under tilted angles from –10° to 10° was investigated by DEM-CFD coupled simulation and verified by the central composite multi-factor bench test. The simulation results showed that the variation coefficient of consistency of row displacement(CV) fluctuated between 14.11% and 18.96% for the rapeseed air-assisted centralized metering without diffusion ring under the top angle of 120°. When the top angle was 150°, the average CV was the smallest, at 13.65%, indicating poor seed discharge uniformity in the rapeseed air-assisted centralized metering device without diffusion ring. When the diffusion ring was located near the air supply elbow(diffusion ring configuration 3), the air-assisted centralized metering device had the best seed distribution uniformity. With the increase of top angle of the distributor, the CV decreased rapidly with the tilt angle. When the top angle was 150° and 165°, the fluctuation range was 6.51%-8.11% and 6.21%-7.66%, respectively. The multi-factor bench test result showed that the air-assisted centralized metering device with diffusion ring configuration 3 and top angle of 150° had the best seed uniformity with the CV at 6.03%-7.03%, which was close to the simulation test result, with the relative error among –0.31%-2.22%.