Abstract: Austenitic stainless steel is widely used in key components such as nuclear reactor pressure vessels, main pipelines and steam generators due to its excellent mechanical properties and corrosion resistance. In this study, an acoustic finite element model of microcracked austenitic stainless steel is constructed, the effects of excitation amplitude and crack dimensional parameters on the nonlinear ultrasonic parameter are explored, and the nonlinear ultrasonic response characteristics of the cracks are systematically analyzed. The high-frequency fatigue loading cycle is used to gradually accumulate damage in the stress concentration region of stainless steel, forming microcracks and inducing their expansion. The crack depth ranges from 0.63 mm to 4.80 mm, and the ultrasonic response signals of different crack sizes were acquired using a nonlinear ultrasonic testing system. Comparative analysis of the experimental and numerical simulation results shows that the nonlinear effect at the crack interface is the main mechanism for the generation of high harmonics. Increasing the excitation amplitude can significantly enhance the response of nonlinear characteristic signals such as crack opening and closing, contact stiffness change and friction slip, and the nonlinear parameter of the ultrasonic nonlinear response increases significantly with the increase of crack depth.