A two dimensional physical and mathematical model on the heat transfer in the flat plate with rectangular inner defect is presented and is solved by finite difference method, bywhich the temperature distribution rules on the inspection surface by infrared thermography are investigated withdifferent defect dimensions, depths and effective thermal conductivities. In addition, the effect of the convective heat transfer in the defect on the temperature distribution of the inspection surface is also studied by building a one dimensionalmodel. From these investigations the conclusions can be drown that the maximum temperature difference increases with the increase of the defect thickness, thermal conductivity difference between the p latmaterial and the defect, but the maximum temperature difference hasa extremum with the increase of the defect length or the dep th of the defect. The convective heat transfer becomes effective when the defect thickness is about 10mm, which changes little with different plate materials and surface convective heat transfer coefficients.