As one of the core working parts of automatic transplanter, the planting mechanism directly determines the effect of transplanting seedlings. The problem of different plant spacings of different crops was focused on. With the selfmade planting mechanism with planetary gears as the research object, the composition and working principle of the planting mechanism were clarified. On this basis, a theoretical model of the planting mechanism was constructed, and the mathematical model of the trajectory about the planting points on front and back duckbills were analyzed and established. The mathematical model was programmed by using Matlab to establish its analysis program. Computeraided optimization of humancomputer interaction interface was developed based on the Matlab GUI platform. With the aid of the optimized design interface, the mathematical model of the trajectory on planting points was solved to simulate the trajectory curve of the planting points on front and back duckbills. The effects of changes in the main parameters of the planting mechanism such as the rotation radius of planet frame, the length of the duckbill, the angle between the initial position of the duckbill and the horizontal line, and the distance from the end point of duckbill to the center of the rotation on the trajectory of the planting site were analyzed. With the value of characteristic parameter λ being greater than 1, the height of the buckle on the planting section being greater than the height of the pot seedling, the front and back duckbills without dragging and hanging seedlings, the size of the hole formed by duckbills on the ridge surface being relatively small and other conditions as optimization goals, through simulation analysis on the influence of main parameters of planting mechanism on trajectory of planting points, institutional parameters that can meet the planting requirements of 280mm, 310mm and 345mm at the same time were obtained and the planting trajectory of parameter combination was verified under three planting distances by simulation. The cucumber seedlings cultivated by 72hole plug trays were selected as the test objects. With the variation coefficient and planting verticality as the evaluation index, prototype performance tests were conducted on the optimized mechanical parameters. The results showed that the excellent rate of planting verticality on three plant distances was higher than 81%, the total passing rate was over 94.8%, and the variation coefficient of spacing distances was less than 3.2%. This proved that the optimized design of the planting mechanism with planetary gears had good transplanting performance, which can meet the requirements for transplanting of multiple planting distances, and verified the correctness and feasibility of theoretical models and optimization methods. The research results provided an effective method for transplanter design.