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Due to the wide application of relaxed static stability in the field of advanced fighter design and the value of further research on the related active control technology,it is necessary to conduct a more comprehensive analysis on the influence of relaxed static stability on the longitudinal flight performance of aircraft. In this paper,the center of gravity is taken backward as the way to relax the static stability,the relationship between the relaxed static stability and the pitching moment coefficient of a certain fighter jet is analyzed to build the nonlinear model of fighter jets and analyze the open-loop characteristics. On this basis,the influence mechanism of the relaxed static stability on the operational equilibrium of the aircraft is analyzed to linearize the nonlinear model of the fighter aircraft,and the influence of the relaxed static stability on the step response characteristics and the long and short period modal characteristics of the longitudinal linear model of the aircraft is analyzed. The simulation results show that the relaxed static stability can increase the pitching moment coefficient and improve the aircraft performance,but it may cause the modal characteristics of the aircraft body to deteriorate,which needs to be considered in the design of control rate.