In order to solve the problems of low root cutting rate and high bulb damage rate after cutting device in the process of garlic combined harvesting, a root cutting device was designed. The key parts of the root cutting device on garlic combine harvester mainly included clamping conveying mechanism and cutting mechanism. According to the structural characteristics and operation requirements of garlic combine harvester, the bulb and root could be separated automatically in the process of movement with the counter rotating disc cutter and pressing of shifting wheel. The dynamic and deformation model of shifting wheel pair and the mechanical model of root cutting were established to achieve the separation of bulb and root in this process. Taking sprocket speed, shifting wheel speed and disc cutter speed as test factors, taking garlic damage rate and root cutting rate as test index, the mathematical models of response values were established and regression analysis and response surface analysis were carried out. The test results showed that when the speed of sprocket was 107r/min, the speed of paddle wheel was 52r/min and the speed of disc cutter was 197r/min, the performance of root cutting device of combined garlic harvester was the best, with the garlic damage rate of 0.63% and the root cutting rate of 97.07%. The optimization results were verified by tests, and the verification results were basically consistent with the optimization results. The research results could provide a technical reference for further exploring the technology of garlic combine harvester and bulb-root separator.