Aiming at the problem of easy wear of the slideway in the link-slide type seedling picking mechanism, a Watt-Ⅱ six-bar seedling picking mechanism that met the trajectory requirements for automatic transplanting of vegetable plug seedlings was proposed. It adopted a fully hinged linkage structure to avoid slideway wear. Firstly, the trajectory, attitude and working requirements of the seedling picking mechanism in the three processes of seedling picking, seedling conveying and seedling releasing were analyzed, an open trajectory was designed where the forward operational path coincided with the return path. Furthermore, five key poses along this trajectory were identified and defined. Then the Watt-Ⅱ six-bar mechanism was simplified into a double-rocker mechanism and a crank-rocker mechanism. The relative displacement matrix equation of the planar 2R open-chain mechanism was established, and the kinematic synthesis design equations were constructed according to the joint constraints. Through the homotopy algorithm, two sets of mechanism parameters meeting the design requirements were obtained to form the double-rocker mechanism. Combined with the quick-return ratio K, the crank-rocker mechanism was designed, and the two were combined to form the Watt-Ⅱ six-bar mechanism. Finally, the structural design, virtual simulation and prototype test of the seedling picking mechanism were carried out. The results showed that the theoretical trajectory, simulation trajectory and the motion trajectory of the bench test were basically consistent, and the success rate of plug seedling picking was no less than 91.4%, which met the design requirements of the automatic seedling picking mechanism for leafy vegetable plug seedlings. This verified the rationality of the design of the Watt-Ⅱ six-bar seedling picking mechanism, which had a good application prospect.