This research is aimed at exploring the characteristics of a parallel-type hybrid electric system with two-planetary gear trains by means of computer-based simulation and analysis. The highlight of the research is to maintain the optimum operation of the internal combustion engine and improve the output efficiency of the electric motor. Generally, the engine distribution mechanism in this system comprises a secondary battery, electronically controlled modules, two-planetary gear trains, an internal combustion engine and an output axle of the internal combustion engine fitted with a flywheel. When the load of internal combustion engine's output axle is smaller than the designed dynamics, the residual dynamics can be distributed to the generator for generation of electricity, with its aim of maintaining the operation of internal combustion engine in an optimum range. To meet different loads of road surface and requirements of drivers, the desired dynamic power shall be obtained from the control of rotational speed of internal combustion engine and excited current of generator. With two-planetary gear trains matching the controller, this system can yield the output effect from combined dynamics.