The emergence of multi-mode hybrid drive system makes it possible for hybrid electric vehicles to remain high efficiency over a wide Speed Ratio range as well as reduce the requirement for motor capacity. The multi-mode hybrid drive system has been deeply investigated. A mathematical model to evaluate the system economy has been established, and the relationship between motor capacity and planetary gear characteristic parameter has been studied based on the model. According to actual demand, with the purpose of minimizing the motor, the planetary system is matched by making the engine running in economic area, and the motor parameter is calculated.