It is significant to determine the equivalent static load of impact load for the strength design and verification of aircraft structures. The formula to calculate shock load dynamic effect coefficient is established basing on terminal peak saw-tooth shock pulse describing the time history of shock load, single freedom dynamic response model and the equivalent principle of model displacement. Then the function relationship between dynamic effect coefficient and the product of shock load duration and model natural frequency is deduced from trigonometric function inequality. The flow chart to determine equivalent static load of shock load is drawn according to the above theory. Finally this method is used to predict the equivalent static load of arresting hook system and aircraft model ditching process. The comparative results show that the maximal relative error of dynamic effect coefficient of arresting impact load is below 4% between theoretic estimated values and numerical simulated values by FEM, the relative error of equivalent static pressure during model ditching test is below 1.6% between theoretic estimated value and determined design value.