The influence of earth rotation on choice of firing position for motive ballistic missile was studied in this paper. The trajectory model of ballistic missile in free flight phase was established. Under the same initial conditions, calculations were accomplished with and without consideration of the influence of ballistic missile range, respectively. The conclusion is that the influence of earth rotation on the range of ballistic missile varies with the latitude of launch point, the launch orientation and the range of ballistic missile, and the influence is greater with increasing range.