Aiming at the problems of low operation efficiency and poor operation quality of semi-automatic transplanting machine, a stem-type automatic seedling picking device for vegetable transplanting robot was designed. The seedling pick-up device can realize efficient and high-quality automatic seedling pick-up and delivery operation through whole row of seedling pick-up, equidistant seedling separation and precise seedling delivery. The motion mechanics model of multi-stage scissors seedling mechanism and clamping seedling device was established. The model design and analysis calculation of the falling motion and pneumatic system of pot seedlings were carried out, and the seedling test device was built. Plug pepper seedlings were selected as the research object, and the seedling age, matrix moisture content and seedling frequency were taken as the experimental factors. A single factor experiment was designed to evaluate the success rate of seedling and matrix crushing rate. According to the test results, the Box-Behnken response surface analysis method was used to design the orthogonal experiment. The effects of the interaction between seedling age and substrate moisture content, seedling age and seedling picking frequency, substrate moisture content and seedling picking frequency on the seedling picking effect were explored, and the seedling picking parameters were optimized. The results showed that when the seedling age was 33d, the water content of pot seedling substrate was 46%, the frequency of seedling picking was 75 plants/min, the success rate of seedling picking was 97.36%, and the substrate crushing was 5.07%, which could meet the requirements of seedling picking and throwing of field automatic transplanting.