The hardened layer depth (HLD) of metal parts is one of the important factors to determine its wear resistance and fatigue strength. Ultrasonic backscatter method can realize nondestructive testing for HLD. However, for those induction hardened parts with large transition layer, it is difficult to accurately locate the limit hardness point, which leads to inaccurate testing results. In this work, HLD measurement experiments of induction quenched 35MnB alloy were carried out. The wavelet multi-resolution analysis (MRA) was employed to find the characteristics of the time-frequency domain of ultrasonic backscattering signals at different decompsosition scales. The energy distribution in each frequency band was analyzed, and the trend characteristics of ultrasonic backscattering signals were extracted to locate the limit hardness point in transition layer of the induction quenched samples. The result shows that the original backscattering signals contain obvious rapidly-oscillating components with low energy and irregular distribution. The signal trend varies slowly with the amplitude of the oscillation with energy portion as high as 96. 73%, which is significantly higher than other decomposition terms. The MRA process essentially filters out most of the clutter noise components irrelative to the depth information of the hardened layer. The actual measurement result shows that the maximum average error of this method is 0. 123 mm, the maximum repeatability error is 6. 24%, showing that the present method achieves high accuracy and repetitive reliability. Compared with metallography and microhardness, this method is more efficient and nondestructive, which shows a good practical application prospect.