Based on different planting conditions and sowing time in the same place and according to different requirements of rape seeding density,the mechanism for variable sowing on 2BFQ-6 rape precision combined direct planter has been designed, depending on the theory that adjusting the engaging relations of gears can change the input shaft’s speed of the metering device. Its structure and working principle were studied in this paper, and it has the regulatory function with four planting densities. At the same time some main structural and performance parameters were analyzed and presented. Modal analysis showed that the rigidity of the output gear shaft is sufficient. The bench tests approved that the error ratio between the sowing amount of the metering device when the mechanism for variable sowing is not installed and the one when it is installed is less than 7% within the same time and under the same condition. It can satisfy the requirements of rape’s variable sowing. 