Aiming at the cumbersome operation of soybean plot breeding and replacement operations, a four-bar translational soybean plot breeding and metering device was designed. The parallel link mechanism and the rotatable seeding disc were used to complete the cleaning and changing operations. The key components of the whole machine were designed according to the requirements of soybean plot breeding operations. The relationship between the lower limit angle of soybean filling and the rotation speed of the seeding disc was determined through theory, and the numerical values of the structure parameters such as the diameter of the seeding disc, the size of the hole, the number of the hole, and the angle of the brush of the seed metering device were also determined. The discrete element software was used for simulation to simulate the seed metering process of the seed metering device. Firstly, the relationship between the rotation speed of the seed metering disk and the lower limit angle was analyzed and verified, and then the influence of the thickness of the seed box on the number of residual seeds was analyzed based on this experiment. The test results showed that the lower limit angle was gradually decreased with the increase of the rotation speed of the seeding disc, and the number of residual seeds was firstly decreased and then increased with the increase of the seed box thickness. The lower limit angle of 119° and the seed box thickness of 11mm were determined as the optimization of the seed metering device structural parameters. The field test under the optimal parameters showed that the seed metering device was smoothly cleaned and replaced under the theoretical seed spacing, and there was no seed residue in the seed box after the operation. At the same time, the average qualified index of the seed spacing was 87.61% at the operating speed of 3.6km/h. The rebroadcast index was 6.63% and the missed broadcast index was 5.75%, both of which reached the standard of excellent products.