Abstract: This paper studied on the trajectory planning for a generalized load simulator (GLS), which works with a structured spatial compliant mechanism. Math model of masterslave dualparallel GLS system was established. An optimal trajectory planning method was proposed for the purpose of least wrench error. This method decomposes the optimal trajectory into static optimal trajectory and dynamic complementary trajectory. The former determines the mapping relation from wrench to twist via Hook’s law. The latter, which is used to eliminate the influence of iterative load, is computed by the linearized dynamic equation of the former one. The results of simulation show that this method is capable of generating load trajectory with high precision, and the wrench error is limited within 0.9‰. This work is of great importance to practical engineering.