Abstract: The principle of detecting the lack of external thermal insulation of exterior wall by Infrared thermography was introduced, and the basic steps for ANSYS finite element numerical simulation was discussed. Three dimension heat transfer model of exterior wall which contained the lack of thermal insulation was established. The feasibility of the model was verified by infrared thermographic experiments. Considering the actual situation of the lack of thermal insulation, the optimal testing time of infrared detection in different conditions combined with defect parameters, orientation of wall and seasons was analyzed by numerical simulation, providing suggestions of detecting thermal irregularities for the quality control agencies.