Automatic supplying rape seed system for plot seeder was designed. It took electromagnetic force as power source and went with plot seeder. Key parts of structure and motion were analyzed. Automatic supplying seed system was able to accomplish supplying seed for 34 plots. By the supplying seed experiment without interruption at the different supply amount in the given time, the result showed that adjusting value and uniformity CV decreased with increasing of time. The uniformity CV was at the minimum range when the time was 8s and the minimum uniformity CV for rape seed was 7.59% at supply amount of 7 g. Loss rate for rape seed was 0.97% and error of supply seed was less than 3 mm. Compared with manual operation, the uniformity CV between the two metering devices was blow 1.5% for rape seed.