In response to the current problem that dryland vegetable pot seedling transplanting machine mostly applies to large plant spacing (more than 260mm) transplanting, and there is a large height difference between the seedling catching and planting position, as well as a poor transplanting performance, the high catching and low planting type differential variable attitude planetary gear system planting mechanism was proposed. Based on the agronomic guidance for small and medium plant spacing of vegetable pot seedling transplanting and transplanting mechanism design requirements, the working principle of the double planetary carrier differential planetary gear system planting mechanism was analyzed and its kinematic theoretical model was established. A computer-aided analysis and optimization design software for planting mechanism based on Matlab GUI was developed by combining the objective function constructed with the proposed optimization objectives. A set of optimal mechanism design parameter combinations was obtained through human-computer interaction. The correctness and rationality of the mechanism were preliminarily verified through 3D modeling and assembly, as well as virtual simulation by ADAMS software. The experimental research on the physical prototype of planting mechanisms and the development of test bench systems were carried out. Through idle testing, the consistency of trajectory and attitude of planting mechanism among the actual operation, virtual simulation, and theoretical design was verified. The experimental research on the seedling catching and planting performance of planting mechanism was conducted. The results showed that the planting mechanism was excellent in all experimental indicators of seedling catching and planting, which could meet the expected design requirements of planting mechanism and the standards of dry land transplanting machinery, and it verified the feasibility and practicality of the planting mechanism.