Due to relaxed static stability are widely used in the field of advanced fighter aircraft design, in this paper, based on the related data of a certain type of advanced fighter aircraft, The influence of relaxation of static stability on the total pitching moment coefficient of this type of fighter is analyzed. A aircraft six degrees of freedom nonlinear mathematical model is constructed , whose open-loop characteristics which is effected by the relaxing static stability is analyzed .Secondly, the influence of the relaxed static stability on the aircraft motion equilibrium point is analyzed, and the nonlinear mathematical model of the aircraft is linearized based on the small disturbance theory. On this basis, the influence of the relaxed static stability on the step response characteristics and the long and short periodic mode characteristics of the longitudinal linear model of the aircraft is also analyzed .The influence of relaxed static stability on the longitudinal flight performance of aircraft is analyzed from the above multi-mode and multi-angle.