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In view of the disadvantages of complex transmission system and obvious difference in seedling emergence effect of each row when single metering device is used in precision compound seeder for Pakchoi, a positive and negative pressure combination metering device for Pakchoi with two plates which can simultaneously sow eight rows was designed. The working process and principle of the metering device and the main structural parameters of the metering device were described. The structural parameters of seed tray and seed metering port and the seed migration trajectory were determined by theoretical analysis. By using DEM-CFD gas-solid coupling dynamic grid model, the effects of seed metering disc speed, air chamber negative pressure and air chamber positive pressure on seed metering performance were analyzed. The results showed that negative pressure had a significant impact on seed metering device qualified index, multiple index and missing index. When the seed metering disc speed was 30r/min, the negative pressure was -3000Pa, the positive pressure was 300Pa, the average qualified index of each row was 93.12%, the average multiple index was 3.59%, and the average missing seeding index was 3.29%. The bench test was carried out by using JPS-12 seed metering device detection test-bed. The results showed that when the rotation speed of seed metering disc was 30r/min, the negative pressure was -3000Pa, the positive pressure was 300Pa, the average qualified index of each row was 91.32%, the average multiple index was 6.19%, and the missing seeding index was 2.49%. The field experiment was carried out with a better combination of factors. The results showed that the average number of seedlings of Pakchoi was 10 in each meter, the average plant spacing was 100.48mm, and the uniformity coefficient of variation of the number of seedlings in each row was 8.05%, which met the agronomic requirements of Pakchoi planting, and the research result can provide a reference for the optimization of the structure of precision metering device.