Abstract: Taking 304 austenitic stainless steel samples with different grain size as research object, water-immersed shear wave testing was carried out with a ultrasonic transducer with frequency of 5 MHz. The energy characteristics of crack defect and grain reflection signals were analyzed by fast Fourier transform (FFT) to eliminate the interference of defect signals. Relative second-order nonlinear coefficients of samples without crack defects were obtained by ultrasonic testing, and the relationship between the coefficient and the average grain size was analyzed. The results show that when the energy proportion in 1.6-4.4 MHz band was less than 1%, we considered that no crack defects existed in the austenitic stainless steel. The relative second-order nonlinear coefficient decreased with the increase of average grain size of the austenitic stainless steel, so the parameter could be used to charaterize the grain size of austenitic stainless steel.