Abstract: An air-suction metering device has been widely used to improve the performance of large-size seed crop planting. Among them, the flat seeds cannot fit with the suction hole in an air-suction metering device, due to the irregular structure and large difference in size. The low stability of adsorption can lead to low seeding performance in this case. There are the horizontal and vertical adsorption postures of the flat corn seeds and the suction surface in the traditional air suction seed metering device in the pre-test. The adsorption posture of the seeds has also posed an important influence on the filling and seeding performance. Specifically, the higher the horizontal adsorption ratio is, the better the filling and seeding performance is. In this study, a new air-suction metering device was designed with an inclined convex structure for the seed picking, in order to adjust the adsorption posture of flat corn seeds. A stable adsorption posture was then reached to separate from the population for high seeding accuracy. The corn seeds were first graded, according to the triaxial dimensions. The flat seeds were then selected to divide into circle and length flat forms. A theoretical analysis was also carried out for the operation process of the inclined convex seed metering device. A force analysis was performed on the flat seeds and convex collision, in order to clarify the influence mechanism of the inclined convex on the posture of the flat seeds during the filling process. Three types (plane, groove, and convex) of discs were utilized for the discrete element simulation. Simulation results showed that the convex disc was achieved in the maximum disturbance ability, dispersion degree, and torque, compared with the rest. The tilted convex plate was used to change the attitude of the seed during the seed picking. The parallel adsorption was then realized between the flat surface of the seed and the suction surface. A bench test was carried out under different operating parameters. The results showed that the single adsorption rate of the convex disc was significantly better than that of the groove and plane disc. Moreover, the single adsorption rate first increased, and then decreased with the increase of forward speed and negative pressure, reaching the highest value of 91.70% at 8 km/h, and 90.84% at 3 kPa, respectively. The stable and parallel adsorption rate of the convex disc decreased slightly with the increase of the speed, whereas, increased gradually with the increase of negative pressure, where was tended to be stable at 3 kPa. The adsorption performance of the convex disc was better than the rest. At the same time, the parallel adsorption rate of the three seed discs was also proportional to the stable adsorption rate. Specifically, the qualified rate of the convex disc was stable at 98%, and then gradually decreased to 93.18% at 12 km/h, particularly at the forward speed of 4-8 km/h. The field tests showed that the lowest qualified rate of the convex disc was 90.34%, and the highest missing rate was 6.52% at the speed of 12 km/h. The convex-type seed metering device can fully meet the requirements of precision sowing. The inclined convex seed-picking structure can be expected to effectively adjust the adsorption posture of flat seeds for better adsorption stability and the working performance of the seed-metering device. This finding can also provide the technical reference for the high-speed and accurate seed-picking of flat seeds.