Abstract: The theory and method about determination of spacecrafts orbit using the GPS is discussed detail in this paper.The this includes these aspects:1)the special factors on the constellation selection of the low orbit satellite,2)determination of the orbit using the Least Square method,3)Kalman Filter model composed of nine states for the low orbit satellites.All of these aspect were simulated on the computer AST PA4/33. Baced on the particular raquests of the electrical power and reliability of the user'S satellite,a practical method of the orbit determination is put forword in this paper. According to the results of simulation, the accuracy of the method in this thesis satisfies the user's request.The method can be used in practical application.