In orbit determination, the precision ephemeris and state transition matrix are usually obtained by solving two groups of ordinary differential equations with numerical integration method due to the complexity of the force models. A kind of simplified analytical method to compute the state transition matrix is given. The method is not only very efficient for the case where the orbit arc is not too long, but also can avoid the integration of two groups of ordinary differential equations at the same time. Some practical test examples also show the efficiency of the method.