Abstract: In order to apply the eddy current method to nondestructive test of the surface hardness of mould steel, the correlationstudies were carried out from both experimental method and the sensitivity analysis of sensor. Firstly, the samples of 45 steel and S136 steel with different hardness were prepared by adopting different heat treatment processes, and the relationship curves between hardness and tempering temperature were obtained. Secondly, the eddy current test system was used to test all the samples, and the impedance amplitude and hardness of the samples were analyzed. The results show that good linear relation between the impedance amplitude and hardness of 45 steel, while good nonlinear relation between the impedance amplitude and hardness of S136 steel, Finally, the sensitivity of three different sensors for detecting hardness were analyzed, and the detection sensitivity was higher when the coil inductance was larger. Moreover, electromagnetic sensor was produced and the relevant test platform was built. The hardness of 45 steel and S136 steel were tested, the test results are consistent with the results of the conventional eddy current sensor. The reliability of test methods and data processing method are verified. The above studies show that eddy current testing can characterize the mechanical properties of mould steel fast and nondestructively, and the eddy current testing method and the system are expected to be applied to engineering quality testing.