According to agronomic requirements, most leafy vegetables adopt a dense transplanting mode with a plant spacing and row spacing of less than 200mm. However, the automatic seedling picking and throwing devices developed domestically are mostly suitable for transplanting with large plant spacing and row spacing. The gantry arm type seedling picking and throwing device can replace manual operations, but due to the long movement path of the seedling picking claws, the speed of seedling picking and throwing is slow. Therefore, this structure cannot meet the requirements of dense transplanting of vegetables. For the A5-1200 semi-automatic high-density transplanter, an oblique automatic seedling picking and throwing device was designed to address the issue of low transplanting efficiency. The device consisted of a seedling tray displacing mechanism, a picking and throwing arm, and eight claws. The seedling tray and the arm were both arranged at a 45° angle to the horizontal direction, and the claws moved back and forth in a straight line between the seedling picking position and the feeding position, shortening the seedling picking stroke. The installation positions of the claws on the picking and throwing arm were fixed, corresponding to the distance between the seedlings and the distance between the seedling feeding cup, omitting the seedling separation step. The orderly and smoothly operation process was ensured by the PLC control system. The ball screw module was driven by the closed-loop stepper motor in the transmission parts for both the picking and throwing arm and the seedling tray displacing mechanism. To realize the whole row interval seedling picking, the tray displacing mechanism completed a single action within 1s, moved the seedling tray in the horizontal and longitudinal directions, and located the position of the seedlings precisely. The influences of the return speed of picking and throwing arm, the insertion depth and the penetration angle of claw on the picking and throwing effect were analyzed. The Box-Behnken response surface experiments were designed to optimize the working parameters. The results showed that when the return speed of the picking and throwing arm was 300mm/s, the insertion depth of claw was 31mm, and the penetration angle of claw was 10°, the actual success rate of picking and throwing was 97.0%. Equipped with the oblique automatic taking and throwing device, the A5-1200 transplanter had a theoretical transplanting capacity of 7200 plants/h, which met the technical requirements of dense transplanting.