Abstract: 　　　　To get methods avoiding system resonance, a practical analytical model of a helicopter's mechanic system of torsional oscillation was developed. The torsional system consisted of the helicopter's propulsion, the transmission system, the rotor and the tail rotor. The modal properties including modal frequencies and mode shapes of the torsional oscillation system were calculated and analyzed with eigen analysis and impedance matching method. The coupled modal properties were analyzed and compared with the hub fixed isolated rotor and the coupled rotor/propulsion/transmission system. The influence to the isolated blade and the rotor of coupled torsional system though changing rotor speed was gotten. Some effective methods to avoid resonance in designing were given.