A three-dimensional physical and mathematical heat transfer model was built based on the heat transfer theory and the modified Levenberg-Marquardt method.An inverse heat conduction problem was studied based on the modified model.And the computational results show that the new model converges better than the old one.The effects of position,size and the thermal conductance of the defect on the computational results were analyzed according to the different defect.And the results show that the modified model is more suitable to calculate the position,size of the defect and the thermal conductance of the defect respectively than together.And the results are affected by the temperature measurement errors,especially the thermal conductance of defect.