The radiation heat load of the furnace is an important thermal characteristics of the boiler, which is of great significance for combustion adjustment and water wall monitoring. However, direct measurement faces difficulties such as system complexity, high installation cost, operation and maintenance difficulties, and damage the original water wall structure. This paper uses the finite volume method to numerically simulate rectangular fins, and calculates the distribution of metal temperature field on membrane water-cooled walls under different operating conditions of wide load range. By studying the relationship between the temperature differences among the characteristic points of the water-cooled wall and the radiation heat load of the furnace, it was found that the temperature difference between the fin root and tip on the backfire side is hardly affected by the heat transfer coefficient of the working fluid inside the tube, instead, it is directly proportional to the radiation heat load of the furnace with a unique coefficient related to the fin structure, physical parameters, and the position of the two points. This method is used for the prediction of the radiation heat load of the furnace, and verified by experiment results that have been done. The results show that this method has high prediction accuracy, low measurement cost, and is easy to install and maintain.