Seedling pick-up mechanism is the core mechanism of dry land automatic transplanting machine, and it replaces the traditional manual labor, which picks up seedlings from seedling box folder and sends the single seedling to the planting machine. The dry land automatic transplanting machines in Japan, South Korea and other countries adopt to bar and slider mechanism scheme or adopt chute in combination mechanism scheme, but they lack efficiency. China scholars do a lot of researches on making use of non-circular planetary gear train to achieve special non-circular trajectory, and this kind of transmission is a potential type of effective seedling pick-up mechanism, but it's very difficult to design non-circular planetary gear train mechanism when it is oriented to complex trajectory and needs to meet the attitude requirements, and the corresponding research is still not sufficient. Reverse solution is an effective way when oriented to mechanism design of complex work requirements, and the reverse design on how to plan the expected trajectory to meet the work requirements as well as the transmission ratio requirements, is the difficult points. Authors have conducted reverse design research on plug seedlings picking-up mechanism and proposed reproduced trajectory judgment criterion based on the monotonic mechanism of output member's relative angle. As to the expected trajectory, the applicability of the non-circular planetary gear system transmission can be judged by the monotonicity of relative angle curve. In this paper, the setting method of the key point in planning the expected trajectory is researched so as to facilitate the design of transmission ratio of non-circular gear, and the determination process of data point is given, which satisfies the requirement of working trajectory and attitude; through compiling mechanism's design software, a set of mechanism parameters are determined, and the simulation is carried out by ADAMS and the result is compared with that of the original design. The results achieved by this article are: the minimum distance between trace and seedling transport box is 21 mm, the minimum width of beak dimension is 2.07 mm, the inclined angle of beak shape is 136.4°, the winging angle of picking-up seedling arm is 24.3° in the feeding periods and is 3.6° in the periods of leaving seedling box, the angle of pushing seedling is 76°, the module of gear is 2.53 mm, height of trace is 285 mm, and the maximum transmission ratio of non-circular gears is 1.81. There are some improvements in the results of new design: the minimum width decreases by 3.88 mm in the seedling period; the winging angle of picking-up seedling arm decreases by 2.43° in the feeding period, which is thus useful to picking-up seedling and transplanting seedling; the distance between the trajectory and the delivering plate increases by 8.75 mm in the return period, which improves safety margin between taking seedling mechanism and delivering device; the highest point of trajectory drops by 42.67 mm, the lowest point of trajectory drops by 3.89 mm, the largest angle of picking-up arm drops from 62.21° to 55.47°, and the maximum transmission ratio of non-circular gears decreases from 1.92 to 1.81. The research in article provides a better choice for the picking-up seedling mechanism, and the reproduced trajectory judgment criterion and the trajectory planning method can give a reference for the design of non-circular gear planetary system.