The external insulation layer of a building's exterior wall is widely used for energy conservation in buildings. An infrared imaging detection method based on a three-dimensional heat transfer model for the surface cracks of the external insulation layer is proposed to effectively detect the quality problems of the external insulation layer. First, with the help of infrared thermal imaging technology, an experimental platform for surface crack detection of the external insulation layer of building exterior wall was built to detect the surface crack of the external insulation layer. Then, using ANSYS software, the three-dimensional infrared thermal imaging detection model for the surface cracks of the external insulation layer of the building exterior wall was established, the model's feasibility was verified, and the effects of crack size and ambient temperature on the detection effect were simulated and calculated.The results show that the experimental ambient temperature and crack size have the greatest influence on the temperature difference between the crack and non-crack areas of the external insulation layer. When the ambient temperature remains constant, the crack width and thickness grow, as does the temperature difference. With an increase in ambient temperature, the temperature difference also increases gradually. When the ambient temperature is below 10 ℃, the temperature difference changes gently; when the ambient temperature exceeds 10 ℃, the rate of temperature difference growth increases gradually with increasing ambient temperature.