A double offset disc air-suction seed metering device for soybean was designed. The key parts of seed metering device was designed and optimized based on the working principles of seed taking and seed metering. The single factor experiments and the quadratic general rotary unitized design were carried out with the rotational speed and air velocity as experimental factors and seed missing rate as experimental index. The regression surface was obtained and its mathematical model was built by using the Design-Expert software. The results showed that the air velocity of 220 m/s and rotational speed of 100 r/min were the best combination factors, which leaded to the seed missing rate of 2.72%.