Torsional vibration analysis method for the power split hybrid system with planetary gear was studied. Resonant speed of the powertrain was optimized through the stiffness matching of torsional damper. Parametric vibration model was built to investigate the influence of gear ratio and damper stiffness on powertrain resonance. Analysis results indicated that there was a high resonant speed zone when the ratio was controlled around some special value. The resonant speed could be reduced out of the engine working speed range with lower stiffness torsional damper.