Abstract: In the process of orbit transfers, propulsions of engine are treated as velocity impulses for their short periods. Extremal Curve Method, was first presented by Breakwell, is a method to establish state equations described by generalized orbital elements after a velocity impulse, and to build extremal fields from initial transfer point base on Modern Control Theory. This method can be used in the case of multi-impulse orbital transfers and space orbital transfers. Dynamics equations of an impulse orbital transfer are introduced in this paper.