Abstract: Automatic transplanting of plug seedling has limited for the two challenges. One is that the broken pot can be caused by the weak twining force between roots and soil, when the end of grippers is inserted into the pot to pick up the seedlings; Two is that the low success rate of seedling picking in the stem-pulling seedling picking mechanism, when the seedling claws clip and pull the stem, due to the weak adhesion force between the pot and the plug. In this study, a seedling picking device was proposed with ejecting pot-clamping stem. The picking up seedling was as follows: The pot was ejected by rod to clip out from the seedling, and then the stem was clamped by seedling clip, while seedling was pulled out of plug tray, seedling shift and releasing clip throwing. A series of tests were carried out on the structure design, the device trial and optimization. Taking "Zhongnong Luheng line pepper 363" plug seedlings aged 35 to 50 days as the object, the stem of plug seedling was tested to obtain the characteristic parameters, stem tensile and stem radial compression mechanical properties; The pot of plug seedling was tested to verify the parameters. The result shows that the maximum drawing force was 3.54-5.38 N, the elastic modulus of the pot was 30.64×10⁻³ MPa, the deformation at the yield point of the pot was 10.87 mm, and the pressure resistance was 12.33 N. The path was optimized for the picking up and throwing seedlings. The optimal path of picking up seedlings stretching along Y direction was 33.98% shorter than that along X direction. The stress models of the pot and stem were established during ejecting and clamping, according to the stem and pot physical properties of pepper plug seedlings. The key parameters of seedling picking up device were designed, including the seedling tray conveying device, the top ejecting device, and the seedling picking up mechanism. An optimal combination of parameters was achieved, where the diameter of the ejecting rod was 15 mm, the height of ejecting pot was 10-20 mm, the height of clamping stem was 20 mm, and the width of the clamping sheet and the width of the seedling clip were both 10 mm. Seedling picking up experiment was carried out on the test bench, where the average total height of plug seedling was 216 mm. The pot moisture content, seedling pick-up frequency, and the height of pot ejecting were used as the influencing factors for the seedling pick-up L9(3⁴) orthogonal experiments. The better growth of the plug seedlings was achieved in the picking up from the ejecting pot-clamping stem. The optimal horizontal combination of factors was determined as well. The damage rate of pot was 1.98% and the success rate of picking up seedling was 98%, when the moisture content was 45%, seedling pick-up frequency was 60 plants/min, and the height of seedling ejecting was 10 mm. Field test was carried out at the working speed of 10 ~ 30 m/min and the picking up seedling frequency of 60 plants/min. The average success rate of seeding pick-up was 93.05%, and the qualified rate of plant spacing was 88.17%. This seedling pick-up device shared the stable seedling pick-up and low damage rate, particularly for the transplanting plug seedlings, such as pepper and tomato. The planter can be expected to install the transplanter for the high-efficient picking up of seedling.