Aiming at the problems of unreasonable trajectory, too large hole and poor uprightness of seedlings when the existing seedling planting mechanism is used for transplanting vegetables with small plant spacing, a Stephenson six-bar seedling planting mechanism suitable for transplanting vegetables with small plant spacing and high density was proposed. Firstly, the kinematics model of Stephenson six-bar seedling planting mechanism was established. Based on Visual Basic 6.0, the auxiliary analysis and optimization design software of seedling planting mechanism was developed. The influence of different mechanism parameters on seedling trajectory attitude was analyzed, and a set of satisfactory mechanism parameters were obtained by optimizing human-computer interaction. Secondly, the structural design of the seedling planting mechanism was carried out. The three-dimensional modeling and virtual prototype simulation were carried out. The test bench was built and the idle test was carried out to obtain the actual seedling planting trajectory. The theoretical trajectory, virtual simulation trajectory and actual seedling planting trajectory were compared. The three trajectories were basically the same, which verified the rationality of the design of the seedling planting mechanism. Finally, the seedling planting test was carried out. When the seedling planting test was carried out at different speeds of 20~40r/min, the average qualified rate of seedling planting was 91.7%. After transplanting, the seedling spacing was controlled at about 110mm, and the verticality was close to 90°, which met the requirements of small spacing and high density transplanting, and verified that the seedling planting mechanism had good feasibility and practicability.