The heat transfer boundary conditions of membrane water-cooled walls mainly include the radiation heat load on the water-cooled wall surface, the convective heat transfer coefficient inside the pipe, and the temperature of the medium inside the pipe. Quickly and accurately obtaining these three parameters is a necessary condition for achieving online monitoring of boiler water-cooled wall temperature and ensuring hydrodynamic safety. In view of the difficulties in directly measuring the heat transfer boundary parameters of water-cooled walls due to the complexity of the heat transfer process, this paper develops a method of decoupling these parameters by analyzing the analytical solution of the temperature field of membrane water-cooled walls and numerical simulation results of finite volume method. This method achieves a dimensionality reduction from a three-dimensional heat transfer system to a one-dimensional heat transfer system and provides a simplified calculation model and fast calculation tool. The temperature distribution of the outer wall of the water-cooled wall metal toward the fire side was reconstructed using the predicted heat transfer boundary parameters as a known condition, and by comparing with the results of numerical experiments, the effectiveness of this method in engineering applications was verified. The impact of temperature measurement errors on boundary condition parameter prediction was also discussed.