Focusing on the problem of high missing-seeding rate of commonly used air-suction seed metering device when sowing corn seeds, especially at the edge of fields, a kind of air-suction precision metering device which used a mechanical supporting plate to assist carrying seeds was designed. There were many cells around the edge of the mechanical supporting plate which help to support and grip seeds, so the performance of attaching seeds of the air-suction seeding disc increased remarkably. The parameter of metering-device working area and main structural parameters of the supporting plate were analyzed and determined. Experiment results showed that the seeding quality of the newly designed metering device was satisfactory and the qualified index of seed spacing exceeded 91.40%, the missing-seeding rate and the reseeding rate were less than 3.82% and 4.78% respectively, and the coefficient of variation of seed spacing was less than 18.37%. All of which were obviously better than the corresponding indexes of commonly used air-suction seed metering device under the same working speed and air pressure.