The basic principle of magnetic flux leakage test is introduced and a new characteristic quantity extraction method is proposed based on the existing limitation.This characteristic quantity is not influenced by the width of the defect when testing the depth of the defect.The influence of the change of the depth and width of the defect on the characteristic quantity is comparatively analyzed by finite element simulation，and the defect test is conducted.The experiment results and finite element simulation results prove that the characteristic quantity proposed in this paper is not affected by the defect width and can be used to quantify the depth of the defect.