Aiming at the problem that the qualified rate is not easy to ensure when the seedling guiding tube planting mechanism is planted at high speed, a ditching multi-bar seedling planting mechanism with seedling pushing and supporting function was designed. The working principle of the planting mechanism was analyzed, and then the kinematics model of the mechanism was established. Based on the kinematics model, an aided-analysis and optimization software of the seedling planting mechanism was developed by using Matlab. According to the seedling planting requirements, the optimization objective was established, and the optimal mechanism parameters were obtained by the software and the Matlab genetic algorithm toolbox. With these parameters, the structure design and 3D modeling were completed, and the virtual simulation analysis was carried out to verify the correctness of design of the mechanism. The physical prototype of the seedling planting mechanism was manufactured, and the semi-automatic and automatic vegetable plug seedling planting experiments were carried out. The results showed that when the planting speed was not higher than 90 plants/min, the qualified rates were more than 95%;when the planting speed was 120 plants/min, the qualified rates remained above 89%, which indicated that the mechanism was feasible for the seedling planting operation. The design provided a reference for the study of vegetable plug seedling transplanting machine.