China is the largest vegetable producer and consumer in the world, and seedling transplanting is the main method of vegetable cultivation. To address the shortcomings of the two types of automatic vegetable transplanters, the stalk clamping type and the pot ejection type, which are used to pick up seedlings, an alternating seedling picking and dropping mechanism was designd based on the combined ejecting-clamping picking method for symmetrically arranged bendable seedling trays. The mechanism was driven by the combination of crank rocker and cylinder. After the seedling ejecting device released the seedling from the cavity wall of the seedling tray, the pneumatic seedling clip clamped the seedling from the seedling picking position to the seedling throwing position to complete the seedling picking and throwing operation, and the seedling picking and throwing were carried out alternately on the front and rear sides of the seedling tray, thus enhancing the efficiency. The working principle, key point motion trajectory and structural composition of the mechanism were analyzed. The influence of key factors on the trajectory of seedling clamping point was analyzed and the following values were selected. The drive crank speed was 10r/min, the length of the seedling throwing rocker was 309mm, the drive cylinder extension speed was 25mm/s, the seedling pulling was completed within 0.8s, and the extension moment was 0.4s ahead. Under this combination of parameters, the maximum lateral displacement of seedling clamping point in the seedling pulling stage was 9.6mm, the accumulated lateral displacement was 0mm, and the theoretical lifting height was 44mm, which met the theoretical requirements of seedling picking and dropping operation. The performance test of picking and dropping seedlings under the planting frequency of 70~120 seedlings/min in a single row was carried out with 45d growth time of pepper seedlings as the operation object. The test results showed that the mechanism can achieve 93% success rate of seedling picking, 95% success rate of seedling dropping and 88% overall success rate at planting frequency was 100 plants/min in a single row, which met the requirements of seedling picking and dropping operation. The feasibility of the seedling picking and dropping mechanism was verified.