Abstract: The basis of the theory of magnetic flux leakage(MFL) was introduced. The 3D finite element model was established and the influences of the changes of defect depth, width and lift-off on the defect alternating current magnetic field leakage were studied by way of finite element numerical simulation, and the relation curves were given. The analysis results provided a basis for the testing of the alternating current magnetic field leakeage testing. Finally, the experimental results were presented, and the results from experiment and finite element analysis were in agreement very well by comparison.