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Aiming at the air-fed seed discharging system in the seed discharging volume increases in the seed mixing device is prone to seed reflux, clogging and other phenomena, resulting in the discharge system of high-speed sowing operation of poor adaptability of the problem, the design of a sudden expansion of the Venturi structure of the high-throughput seed mixing device was done, through the theoretical analysis to determine the diameter of the seed tube, the length of the mixing zone, the length of the cross-section of the seed inlet length of the structure of the key parameters. Fluent airflow field simulation results showed that when the wind speed was 12m/s, 15m/s and 18m/s, the minimum negative pressure at the throat of the seed mixing device with Venturi structure was -236Pa, -372Pa and -538Pa, respectively, and the minimum negative pressure at the pre-mixing area of the seed mixing device with abruptly expanding Venturi structure was -396Pa, -621Pa and -1007Pa, respectively, and the “Venturi effect” produced by abruptly expanding Venturi structure was better than that of ordinary Venturi structure. The results of DEM-CFD coupled simulation showed that about 12% of rape seeds bounced after collision with the extended part of the Venturi structure, and after collision, the seeds kept moving towards the mixing area, and there was no phenomena of reflux and blockage in both rape and wheat. The results of bench test showed that the seed mixing device of traditional Venturi structure showed seed reflux at the seed mixing flux of oilseed rape greater than or equal to 2.4g/s, seed reflux at the seed mixing flux of wheat greater than or equal to 60g/s and the coefficient of variation of the consistency of the rows’ displacements were increased significantly, and clogging occurred at the seed mixing flux was greater than or equal to 100g/s;the seed mixing device of the amplified Venturi structure had seed reflux at the mixing flux of oilseed rape smaller than or equal to 4.8g/s, wheat seed mixing flux less than or equal to 120g/s did not appear seed reflux, clogging and other phenomena, oilseed rape, wheat rows discharge consistency coefficient of variation was lower than 5.17%, 3.29%, the total discharge stability coefficient of variation was lower than 1.19%, 0.80%, to meet the oilseed rape, wheat seeding machine can be in the speed of 4~12km/h agronomic sowing requirements, the results of the study for the air-fed seeding system structure optimization can provide a reference. The results can provide reference for the optimization of the structure of the air-fed seed discharge system.