During the vegetable transplanting for the existing vegetable semi-automatic transplanter, the plug seedlings picking and feeding are nearly dependent on manual work, so the work efficiency is very low. Aiming at this problem, a vegetable plug seedling automatic pick-up mechanism was designed. The motion and dynamic mathematical model were established. Moreover, the influences over the mechanism motion and dynamic characteristic by some important parameters were analyzed by using the parametric models. The design of optimization objective for automatic picking vegetable plug seedlings was proposed. And then, the range of mechanism parameter which met the requirements of the motion trajectory for plug seedling picking was obtained. In addition, the dynamic analysis for the pick-up mechanism was carried out based on the range. Meanwhile, taking the minimal force wave of mechanism bearing as optimization objective, the optimal parameter combination of pick-up mechanism was obtained. With these parameters, the plug seedling pick-up mechanism processed better the motion trajectory and better dynamic performance which met the demands of seedling picking.