In order to further improve the planting efficiency of the high-speed automatic transplanter for vegetable plug seedlings, a three-arm gear train planting mechanism was proposed, and an approximate multi-pose motion synthesis method based on genetic algorithm (GA) was introduced. Firstly, with the key pose (position and orientation) data on the ideal planting trajectory as constraints, the approximate multi-pose motion synthesis optimization model of gear train planting mechanism simplified model (planar RR mechanism) was established by the condition of invariable link length, and the optimal structural parameters of the mechanism were obtained by using Matlab GA toolbox. Then, the total transmission ratio of the gear train was calculated and distributed by the motion parameters of revolute joints of the planar RR mechanism, so as to realize the design of gear train planting mechanism. Finally, the structure design, simulation analysis and test verification of the three-arm gear train planting mechanism were carried out. The results showed that the actual motion trajectory and posture of the mechanism were basically consistent with the theoretical design. When the planting frequency was 120 plants/(min·row) and the theoretical planting spacing was 300mm, the planting success rate was 96.7%, the actual average planting spacing was 298mm and the average hole width was 70mm, which met the requirements of high-speed transplanting. The correctness of the proposed method and practicability of the three-arm gear train planting mechanism were verified.