Delta parallel mechanism is effective in speed, kinematic accuracy and flexibility, which is appropriate for the plug seedling transplanting and healthy plug seedling filling, thus a high-speed plug seeding transplanting robot was designed by using Delta parallel mechanism with a pneumatic manipulator. The reachable workspace of the plug seedling transplanting robot was obtained by building the single chain constraint equations of the Delta parallel mechanism;taking the condition number of Jacobian matrix as the penality condition, a dimensional synthesis target function was built considering the size and kinematic performances of the plug seeding transplanting robot for the purpose of making the reachable workspace close to the needing workspace, then the dimensional synthesis for the plug seedling transplanting robot was obtained by using the genetic algorithm. In accordance with the motion demand of plug seedling translating robot, trajectory planning was carried out and quintic polynomial motion law was taken as the principle of operation for the moving platform. Based on the mechanism dimensions and trajectory planning, a physical prototype was built and transplanting experiment was carried out. The experiment results showed that the success rate of plug seedling transplanting was decreased when the transplanting acceleration was increased, the success rate of plug seeding transplanting was up to 95.5%, the success rate of plug seeding filling was up to 92%, transplanting rate can reach 2149 plants per hour when the transplanting acceleration was 30m/s2, which demonstrated the practicability and rationality of the dimensional synthesis and trajectory planning when the Delta parallel mechanism was used in plug seedling transplanting.