Abstract: A theoretical method based on modified Hertz contact theory is presented for calculation of impact force of falling rock. Combining the formula of the impact depth of falling rock in Code for design of highway subgrades. Special attentions are given to the complexity of impact and the thickness of soil buffer layer. The paper introduces Parameters influence coefficientζand polynomial was obtained for polynomial fitting method about it, according to Japan Highway Public Corporation calculated the results go up 40% as the base value. The method of Yang is modified using test results of S. Kawahara, etc. Professor to consider the maximum impact force. Polynomial is obtained for the ratio of high value (h = 0.5m) and low value (h = 3.0m). This method is compared with the method of Chen professor, it is shown that the method of Chen professor is slightly higher than this method in low value, the scope for 1.05 - 1.02 (H = 5 - 30m). A formula is obtained using Boussinnesq s solution for the additional stress of falling rock impact load acting on the retaining wall back, This provides a theoretical basis for the structural design of retaining wall.