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Aiming at the defect that the existing air-suction disc precision seeder needs to replace the seed metering disc for seeding seeds of different sizes, and in order to save cost and improve the versatility of the seed metering device, a variable-size double disc air-suction precision seeder was developed, which can realize the precision seeding of seeds of different sizes without changing the disc. Based on the existing disc-type metering device, a double disc structure with variable particle size was designed, its basic structure and working principle were expounded, theoretical analysis of the working process and key components of the metering device were made, the key structural parameters such as hole layout, hole shape and hole cone angle were determined, and Fluent simulation was used to analyze the influence of five combined holes on the flow field in the chamber. The optimum combination hole parameters were obtained by real analysis, and the uniformity test and orthogonal test of seeding were carried out on JSP-12 seeding test-bed. The reasonable ranges of negative pressure and rotational speed of seeding plate were obtained when the seeding performance was better. The results showed that the qualified rate was 90.46%, the missing rate was 2.59%, and the replaying rate was 6.94% when the disc hole of the metering device was a chamfered hole with conical angle of 60 degrees, the rotational speed was 34.5r/min and the negative pressure was 4.1kPa. The metering performance was better and the seeding requirement was satisfied. During the verification process of field experiment, the average qualified rate was 90.16%, the seeding leakage rate was 2.77%, the consistency coefficient of variation of seeding row displacement was 5.34%, and the stability coefficient of total displacement was 4.86%. Compared with traditional metering device, the operation effect was significantly improved.