A type of rotary seedling pickup mechanism used in vegetable plug seedling transplanter which can realize nonuniform continuous transmission was taken as the research object. In order to improve the working stability of the seedling pickup mechanism, the counterweights method was used to improve the structure of the seedling pickup mechanism to reduce the vibration of the mechanism and improve the performance of the mechanism. The optimization design model of the counterweight was established, and the mass and centroid radius of the counterweight was determined as 0.21kg and 102mm respectively by using the exhaustive method through UG and ADAMS software. ADAMS software was applied to carry out the dynamics simulation and analysis of the seedling pickup mechanism. Counterweight was machined and assembled on the mechanism prototype to conduct dynamics tests. The simulation and test results of bearing reaction force of the seedling pickup mechanism after addition of the counterweight were compared with that before addition of the counterweight to verify the validity of the dynamics performance improvement of the seedling pickup mechanism respectively. The prototype test results showed that the maximal amplitudes of bearing reaction force of the seedling pickup mechanism in x and y directions were reduced from 409N and 77N before improvement to 369N and 69N, respectively, which were decreased by 9.8% and 10.4%. At the same time, the variances were reduced from 5126N2 and 553N2 before improvement to 3391N2 and 398N2, respectively, which were decreased by 33.8% and 28%. The dynamics performance of the seedling pickup mechanism was better and the working stability of the mechanism was also improved.