As the scale of agricultural production gradually expands, agriculture is developing forward to mechanization and intelligent direction. At present the main ways of domestic thinning are artificial thinning and chemical control thinning. Artificial thinning has the disadvantages of low efficiency and high labor intensity. Chemical control thinning makes seedling emergence come out not evenly, and people have to fill the gaps manually. Mechanical thinning can improve the working efficiency and bring great economic benefits with even seedling. The swing motion thinning robot was designed for the first thinning of seedlings and the kinematics model was established for the optimal to simulate the moving path. Then the virtual orthogonal experiment was done by different optimal edge lengths, different angles between optimal edge and optimal handle, different optimal handle lengths and different distances between optimal handle and center of crop-row. The prediction model of weeding rate and wounded seedling rate were analyzed by taking the coverage rate and the inbreaking rate as evaluation indexes. Based on the calculation, the best parameters in combination were as follows: distance between optimal handle and center of crop-row of 55mm, optimal edge length of 75mm, optimal handle length of 130mm, angle between optimal edge and optimal handle of 65°.The experiment result verified that the simulation and orthogonal experiment were accurate and efficient with good operation effectiveness of the prototype, and the weeding rate reached 89.4%, yet the wounded seedling rate was only 7.2%.