The influence of pitch/lag coupling on helicopter air resonance is investigated by the methods of multi- blade coordinates transformation and complex coordinates transfer in this paper.  First,  helicopter air resonance equations of motion considering pitch/lag coupling is derived,  related analysis model and calculation programs are built to study the effect of pitch/lag coupling on helicopter air resonance through the eigenvalue of the system.  Second,  through the eigenvector and the analysis of the approximate expression of mutual excitation of different degrees of freedom,  the explanations of the helicopter air resonance considering pitch/lag coupling are explicated.  The results indicated that the positive pitch/lag coupling can restrain the instability of helicopter rotor/body coupling system and the system becomes strongly stabilized in the case of considering flap/lag structure coupling and pitch/lag coupling together.