The skin depth of metal materials is very small under high-frequency eddy current incentives,so it can be regarded as surface heating.As it is difficult to capture subsurface defects of the specimen,a longitudinal heat conduction method based on pulsed eddy current thermography was proposed to identify the subsurface defect.Meanwhile,the subsurface defects were quantitatively studied according to the change of curve slope.For ferromagnetic materials and non-ferromagnetic materials,their subsurface defects were studied by the finite element simulation and experiments.Results show that longitudinal heat conduction is greater than lateral heat conduction when length-width ratio of the defect is greater than 2,and the obvious temperature difference on the surface is formed to realize the detection of subsurface defects.According to time points corresponding to linear and nonlinear changes of temperature response curve slope,the relationship between the defect depths is quantitatively analyzed.