To ensure the consistency of planting depth when facing irregular ridge shape of sloping farmland in hilly areas, domestic and foreign scholars have carried out a lot of research on profiling technology of transplanter components. Aiming at the problem that the passive profiling system is difficult to realize real-time adjustment of planting depth, an electric planting mechanism which can adaptively adjust the planting depth according to the ridge height was designed in this paper. For the mechanism, kinematic modeling of the mechanism was conducted, and the working principle of the planting system was analyzed. Using the results of the kinematic modeling, a computer-aided analysis and optimization interface for the planting mechanism was developed to study the mapping relationship between structural parameters and the planting trajectory. By establishing an objective optimization function and setting mechanism constraints, a relatively optimal parameter combination was obtained by using the successive approximation method. A simulation test for constant-depth planting was conducted, verifying that the proposed mechanism can adjust the planting depth through five sets of angle variations, while meeting the requirements for the planting trajectory and the velocity at the planting endpoint. A static trajectory test of the mechanism was carried out, showing that the maximum error of the lowest point between the five sets of actual static trajectories and the simulated static trajectories was 6.86 mm, and the maximum error in trajectory height was 13.98 mm, with a high degree of coincidence for each set of trajectories. The constant-depth planting performance test indicated that, at a planting frequency of 15 plants/min and a unit speed of 3.6 m/min, the mechanism can realize stable constant-depth planting operation on both flat and uneven ridge surfaces.