Icing leads to the deterioration of aircraft aerodynamic characteristics， which affects the flight quality. Aiming at the problem of aircraft flight quality evaluation in the full envelope range under the condition of wing leading edge icing， an equivalent system fitting method based on adaptive fitting initial value is proposed. The icing configuration of the wing leading edge is constructed， and the aerodynamic data of the background aircraft are obtained through numerical simulation. The aircraft system model is established and the elevator pulse doubling operation is conducted to obtain the response data of the aircraft. The data characteristics are analyzed to calculate the initial value of adaptive fitting. The equivalent system fitting is carried out under the conditions of clean configuration and different icing severity within the full envelope of the aircraft. The modal characteristic parameters of longitudinal short period flight are obtained， and the corresponding flight quality level is obtained. The simulation results show that icing will affect the modal characteristics and reduce the flight quality of the aircraft， and may lead to the degradation of flight quality in serious cases.