Abstract The quenched SAE 1552 steel has excellent mechanical properties. The depth of hardened layer is closely related to its mechanical properties, so it is of great significance to detect the depth of hardened layer. By using ultrasonic testing technology, the relationship between each ultrasonic parameter and quenchable material with different thickness is established. Through short-time Fourier transform, the ultrasonic signal is analyzed. It is observed that the time-domain waveform of steel with different hardened layer thickness is very different, and the ultrasonic frequency and signal amplitude are also different. This feature can be used as an important theoretical method of ultrasonic testing, which proves that the ultrasonic testing method can accurately detect the hardened metal layer.