A reverse design method for transplanting mechanism of unequal speed planetary gear train based on general spatial continuous closed trajectory was proposed in order to make the planetary gear train transplanting mechanism achieve a complex spatial trajectory and better meet the agronomic requirements for transplanting rice seedlings in wide and narrow rows. First of all, the planetary gear train transplanting mechanism was simplified as a spatial three-bar two-degree-of-freedom (2R) open-loop mechanism, and the ideal transplanting trajectory curve fitted by the cubic non-uniform B-spline curve was used as a constraint to establish a spatial 2R mechanism kinematics model, the formation mechanism of the terminal trajectory was analyzed, the rod length and the relative position relationship between the rotation axis of the mechanism were solved by using the spatial geometric relationship. Secondly, the total transmission ratio of the planetary gear train mechanism was obtained from the relative angular displacement relationship between the input and output axes. The degrees of freedom of the spatial 2R mechanism were reduced by attaching to the unequal speed gear pair. The transmission ratio distribution and the pitch curve of the non-circular gear were designed according to the combination type of the gear pair. Finally, based on the spatial “8” shaped transplanting trajectory, a transplanting mechanism of rice seedling with wide and narrow rows driven by plane non-circular gear and bevel gear was designed in the application example, and the virtual prototype simulation and physical prototype test were carried out. The results showed that the simulation trajectory and test trajectory were basically consistent with the given trajectory, which verified the correctness and feasibility of the theoretical method.