Abstract: Nondestructive testing of the depth of hardened layer for the hardened material is one of the research hotpots. And nowadays, the pulsed eddy current (PEC) testing method has been used in this field for its quantitative measurement of case hardened depth. However, more attentions in the studies were paid to the time domain and its limitations were easily found. Accordingly, a new method is proposed based on the spectrum characteristic. In this method, a two-layer simulation model is established based on the principle of the testing of case depth with PEC. Next, the detection signals at different case depths are analyzed in time domain and frequency domain, respectively, and frequency is used as a new frequency spectrum characteristic value to quantitatively determine the depth of hardening zone in frequency domain. Then, the frequency is applied to calculate the case depth. The results show a good linear relationship between the frequency and case hardened depth. Finally, the simulation results are validated by the experimental method. The experiment proves the correctness of the proposed characteristic parameters.