The results of classical Tikhonov regularization method to identify the impact force will always be stable signals. To solve this problem, this paper proposes a semi-cosine function fitting method (SCFF) which uses semi-cosine functions as the basis functions to fit impact forces, and combines genetic algorithm to optimize the parameters; The mathematical solution expression can be obtained without iterative calculation, and the calculation efficiency is high. In this paper, the numerical results of a truss structure show that SCFF method will obtain accurate solutions and it has good robustness which illustrates that SCFF method owns great engineering application prospects.