Abstract: The fatigue damage testing of T700 carbon fiber composite under different stress ratios was carried out, and the nonlinear coefficient values in the fatigue damage process of the composite under different working conditions were obtained by using the nonlinear ultrasonic testing technology to measure the specimens which were applied with different fatigue cycles. It demonstrated that when the fatigue life was about 50%, the relative nonlinear coefficient was sensitive to the change of mechanical properties of materials. It can be used as a characteristic parameter for evaluating fatigue damage of materials.