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In order to solve the problems of low seed feeding accuracy and unstable seed feeding caused by the fast seed conveying speed of the air-fed rice-wheat dual-purpose high-speed seeder, a seed reducer based on the principle of cyclone deceleration was designed. The CFD-DEM coupling simulation method was used to carry out a single factor test to determine the main structural factors and select the appropriate size range. In order to determine the structural parameters of the seed reducer, the Box-Bhnken orthogonal combination simulation test was carried out based on the single factor test results, and the results showed that the optimal structural dimensions were 82.352mm of cylinder diameter D, cylinder length Ht of 101.364mm, exhaust port diameter DP of 25.0002mm, cone length Hz of 67.9025mm, and the indica seed outlet flow velocity V1 and seed vertical velocity V2 were 5.212m/s and 0.462m/s, respectively. The seed flow velocity V1 and seed vertical velocity V2 of japonica rice were 5.339m/s and 0.473m/s, respectively. The flow velocity V1 and seed vertical velocity V2 of wheat seeds were 5.341m/s and 0.408m/s, respectively. The results of bench verification test showed that the vertical velocity of indica rice seeds was 0.411m/s, japonica rice seeds was 0.452m/s, and wheat seeds are 0.457m/s at the seed outlet, which was consistent with the simulation test results. It can be seen from the bench test of strip sowing performance that the effect of strip arrangement with seed reducer was significantly better than that without seed reducer, and the coefficient of variation of displacement uniformity of indica, japonica rice and wheat was decreased from 41.61%, 25% and 37.84% without seed reducer to 9.10%, 8.42% and 8.49%, respectively, which met the performance requirements of seed reducer. The results can provide guidance for improving the seeding performance of air-fed rice-wheat seeder in the future.