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The suitable working negative pressure of pneumatic seed metering device is related to working speed and seed size. However, the working pressure of the seeding system of the existing pneumatic seeder is frequently set to a fixed value which cannot be optimized and adjusted timely. A pneumatic adaptive seeding system was designed based on the positive and negative pressure combined small particle size seeds metering device. The system was controlled by STM32 single-chip microcomputer. By adjusting the working speed of the metering device with speed and monitoring the seeding performance in real time, the working negative pressure of the metering device can be dynamically adjusted, and the actual working negative pressure of the metering device can be continuously maintained as the optimal value under the corresponding working conditions, so as to realize the optimal control of the seeding performance. The results of the bench test showed that the qualified index of the pneumatic adaptive seeding system was greater than 92% and the missing index was less than 6 % under different operating speeds and seed sizes. Compared with the fixed air pressure setting and the open-loop control air pressure adjustment method, the qualified index of seeding was increased by 9.02 and 3.84 percentage points respectively, the multiple index was decreased by 8.44 and 1.99 percentage points respectively, and the missing index was decreased by 0.58 and 1.86 percentage points respectively. Field test showed that the stability coefficient of plant spacing of the seeder equipped with a pneumatic adaptive seeding system was 14.27%, and the consistent coefficient of variation of the number of seedlings in each row was 7.03%. The findings can provide a technical reference for the improvement of the continuous and stable single-grain precision seeding ability of the pneumatic seeder.