An automatic rotary seedling pick-up mechanism used in vegetable pot-seedling transplanter, planetary gear train with eccentric gear and non-circular gear, is taken as the research object to build dynamics model. The dynamics sequence-solution method was used for solving the dynamics model. Then the variation law of force between the rotary angle of planetary carrier and various positions of the mechanism in a working cycle was obtained. The positions included chain, axle center of the sun gear, axle center of middle gears, axle center of planetary gears, meshing point between the sun gear and middle gear, meshing point between the middle gear and planetary gear. Physical prototype of the seedling pick-up mechanism was designed. Dynamics bench test was carried out to investigate dynamics characteristics of the physical prototype under four different rotary speeds, while correctness of dynamics model was verified.