Because the relationship between dependent differential and independent differential cannot be expressed by explicit functions, the design optimization problem of planar torsional springs is expensive black box problem.Differential evolution algorithm is a method used to solve expensive optimization problems in recent years.In this algorithm, the proxy model is used to predict the relationship between fitness and dependent differentials, thus the calculation cost is greatly reduced.A design optimization method based on differential fidelity surrogate model and differential evolution (VFS-DE)was proposed.The proposed VFS-DE was used to test the related optimization function. The results show that the function optimization problem can be effectively solved by using the proposed method.VFS-DE was applied to the design of plane torsion spring of machine tool servo motor, and the effectiveness of this method was verified by relevant experimental results.