Abstract: To tackle the great difference in strength and longevity of gears in the multi-power split planetary gear transmission, an equal strength optimal design method was proposed to assure the minimum volume and safety coefficient difference in the planetary gear transmission system. The load sharing coefficient and dynamic load coefficient of the system was calculated based on the dynamics, the influence of stress cycles on the allowable stress was considered, and the safety coefficient for the strength of gears was calculated based on the AGMA standard. The five power split star gears system was taken as an example to calculate the gear parameters. Result shows that the gear parameters obtained through the optimum design method aiming at the minimum volumes and safety coefficient difference can make the strength of the gear close and effectively reduce the volume of transmission system, which can make the design parameters more reasonable and improve the reliability of the system as well as reduce manufacturing cost.