Abstract: The high proportion of Zhundong high-alkali coal blending puts forward a new demand for the traditional problem of boiler ash deposition and slagging monitoring. In this paper, a radiation imaging model was established, and then the Monte Carlo method was used to accurately solve the flame radiation and wall radiation received by the detector, and the criterion for obtaining the wall image through the flame is established. For coal-fired boilers, a medium-wave infrared detector with a photosensitive band of 3-5um can be used for imaging monitoring of the heating surface of the boiler. When the detector is arranged, the high temperature area of the flame should be avoided, and the optical thickness of the flame between the detector and the wall should be reduced. The experimental verification was carried out on a burning furnace. The mid-wave infrared detector was used to obtain the image of the wall which behind the flame, and the temperature of the image corresponding to the wall was measured, which was basically consistent with the measurement result of the thermocouple. This study will provide an important reference for the monitoring of slagging on the heating surface in coal-fired boilers.