Abstract: It is more difficult to investigate the helicopter dynamic stability in forward flight than in hover.The rotor structure model,aerodynamic model and equations of motion were established.The treatment of equations with periodic coefficients and the influence of related parameters on rotor dynamic stability were investigated.The degree of freedom of the dynamic inflow was introduced into the dynamic stability analysis in forward flight.Starting with the analysis of mutual excitation of different degrees of freedom,the physical nature and influence factors on rotor dynamic instability are revealed.