The transmission characteristics of power flow in gear transmission system were researched by signal flow graph. The general representations of XP and PX hybrid systems were given by summarizing the new achievements of hybrid gear transmission systems. According to the structural characteristics of hybrid power transmission system and the calculation principle of signal flow graph, the models of XP and PX hybrid gear transmission system were established. On this basis, after simplifying the calculation, the formulas of calculating the speed, torque and power of XP and PX hybrid power system were gotten, and the feasibility of this method was verified by example.