Aiming at obtaining lantern shape litchi flesh, a pitting machine was devised and made. Cutter shaft is one of the most important part of the machine and its reciprocating motion cause impact load on the machine. Unilateral load on the surface of the groove result in excessive wear of the cutter shaft. In view of the above problem, dynamic characteristics and the optimum design model of the cutter shaft are established. Used nonlinear optimization toolbox of MATLAB, the best structure parameters of the cutter shaft were calculated. The results are: (1) minimum mass of the shaft is 0.6421 kg ,which is lighter 10.07% than before;（2） the best spiral angle of the groove is 38.38°. 3D parameterized models of the cutter shaft were built and their kinematics simulation were made by using Pro/ENGINEER software. The simulation and test results show that the more rotate speed and spiral angle of the groove are, the higher press on the surface of the groove bears. The above results provide basis for improved design of the pitting machine for litchi fruit.