Due to deformability, flexible eddy current sensors are used in nondestructive testing for complex structures. The flexible Koch fractal eddy current sensor consistently detects short cracks in all directions due to the eddy current conditioning effect of its local microstructure. However, the detection of near-surface and internal cracks has not been studied. This article formulates the equivalent circuit model of the difference pick-up sensor. The evolution of eddy current distribution with depth is studied by simulation method. Through the experimental method, the detection effect of the sensor on the prefabricated crack specimen covered with different thicknesses of aluminum plate is studied. The finite element results show that, with the increase of depth, the eddy current density decreases, the concentration decreases, and the distribution pattern is more and more different from that on the surface. The eddy current shape changes from snowflake to circular. The experimental results show that the output signal of the sensor decreases with the increase of the thickness of the covering layer. When the excitation frequency is 10 kHz, the sensor can detect the maximum thickness of the covered aluminum plate when the crack is 2. 0 mm.