Finite element method is used to analyze the two-dimension heat transfer model.The temperature distribution on material surface can be obtained by using ANSYS simulation.The sum of errors function square is defined as the objective function.The errors function is the difference between the calculated values and simulation results of the material surface temperature.Conjugate gradient method is used to optimize this objective function.The simulation results certifies the feasibility of the algorithm.Study shows that the subsurface defect depth and thermal conductivity of materials are important factors that affect the temperature difference of the material surface.The smaller the depth of the defect from the surface is,the greater the temperature difference of the material surface is.With the increase of thermal conductivity,temperature difference of the material surface increases at first and then decreases.This method can obtain more accurate defect location and the defect size.It provides a feasible way for infrared nondestructive testing.