Due to the phenomenon of transplanting arm mud-dumping in the process of transplanting in the existing Bezier gear planetary gear system rice seedling transplanting mechanism, the causes of the problems caused by the operation trajectory were analyzed, and the transplanting trajectory improvement and solution of mechanism parameters were carried out based on the analytical theory of precise multi-position motion. According to the position and attitude requirements of the three actions of the beginning, the end and the pushing of seedlings during transplanting process, the three precise pose points of the constrained transplanting trajectory were determined. The design process of the gear-train transplanting mechanism was divided into two stages: the motion synthesis of the open-chain 2R mechanism (obtained by removing all gears except the output gear) and the design of the unequal speed transmission ratio. Based on the synthesis method of three-position motion generation mechanism with given link length, the curve equations of the center point (the input point of gear train mechanism) and the circle point (the output point of gear train mechanism) of the open chain 2R mechanism were established and the position curves corresponding to the circle point and the center point were obtained when the length of the rod was different. The desired trajectory with key point position information was fitted. The angular displacement of the open-chain 2R mechanism based on the trajectory was solved and its monotonicity was analyzed, solution and distribution of the total transmission ratio of the mechanism were realized. Transplanting mechanism of rice seedling with seven-gear planetary wheel was designed, and the simulation and bench test were carried out. Through the comparison with the theoretical and experimental results of the original mechanism, it was found that the angle between the backhaul section of the transplanting path of the new mechanism and the ground was increased to 90°, the height of the buckle was increased to 68mm, and the distance between the seedling point and the lowest point of the rotation center of the box was increased to 21mm. The transplanting quality of rice seedling was improved.