Abstract: Powertrain system for a series hybrid electric bus is studied. The study includes the performance parameters matching of key components and the "thermostat" control strategy. On the basis of analyzing the fundamental working modes of the hybrid electric bus, Simulink tool of Matlab is used to realize the calculation of the bus under certain drive cycles. The simulation analyzes the working conditions of each key components and the fuel economy of vehicle. The results verify the concept and objective of the design, i.e. extending the working range of the vehicle, acquiring the best fuel economy and the lowest emission. It demonstrates that the simulation research plays an important role in the design of vehicles.