In order to implement fast and real time computation of disturbing gravity trajectory, after the analysis of each frequency space disturbing gravity vector results based on the point mass model, a cubic spline method was presented for the offset approximation according to the four frequency space disturbing gravity, the computing structure become simple; the delay error caused by the program of onboard computer was avoided. The results show that the computing structure is simple; the disturbing gravity trajectory of a missile can be calculated fast and accurately.