Abstract: The electromagnetic field in eddy current detection decays rapidly in space, and the sensor size is an important factor affecting the sensitivity of eddy current detection. A large flux-gate magnetometer was used to achieve the detection of 304 stainless steel at a depth of 20 mm and aluminum alloy at a depth of 16 mm. At the same time, the numerical simulation of the test process was carried out, and it was found that the actual results of B_x signal maximum variation were 33%~44% lower than the numerical simulation results. However qualitatively both of those two results were in good agreement. Numerical simulation shows that the sensitivity of small size and high sensitivity magnetic sensor in eddy current detection was much better than that of induction coil eddy current detector. Small size of the sensor could make up for the disadvantage of magnetic sensitivity. At the same time, the sensitivity demand curve could be drawn according to the sensor size and sensitivity to compare the eddy current detection sensitivity of different sensors. The results were helpful to optimize the design of new eddy current sensors.