Internal defects of decorative layer on exterior wall, including mortar and ceramic tile decorative layers, were detected using thermal infrared imager. Variable characteristics of surface temperature for decorative layer heated by external heating source were studied. The distribution and variation law of surface temperature on decorative layer with time and space were obtained，and the influences of thickness，area and embedded depth of internal defect on image quality were analyzed. It is revealed that the thicker, bigger and shallower internal defect are more easily detected. The surface temperature of internal defect is maximum temperature point of approximate symmetry along with width direction. Based on the test results, the prediction model of dimensions for internal defect was established through statistical analysis technique, which can provide a reference for the related follow-up study and practical project application.