Abstract: Automotive electronic parking gear is mainly made of SAE J403 carbon steel. In its production process, SAE J403 needs high-frequency induction quenching. Due to the gap in material itself or quenching process, different thickness hardening layers will be formed on the gear surface, which affects the material and mechanical properties of the gear.Through the ultrasonic testing device, the signal acquisition of carbon steel with different hardening layer thickness was completed, the spectrum analysis (fast Fourier transform) of the collected ultrasonic signal was done, the fundamental wave amplitude and second harmonic amplitude of the ultrasonic signal were obtained, and the nonlinear ultrasonic coefficient for evaluating the depth of the hardening layer was calculated.The results show that the nonlinear coefficient of carbon steel increases with the increase of hardened layer depth.Therefore, the depth characterization of carbon steel with different hardening layer thickness is realized by using nonlinear ultrasonic coefficient.