Understanding the formation and the flow of molten slag in the water-cooled membrane wall pulverized coal entrained flow gasifier was very important to determine the wall heat transfer and the reactor operation. In the standard model’s CFD simulation, the wall boundary condition was only able to be set as a fixed heat flow, which was not accurate in describing the influence of the temperature distribution and slag distribution on heat transfer under the actual situation. In this work, an extensive and detailed slag model was established and validated. The complete slag model included a particle capture model, wall heat transfer model, and slag thickness model through real-time coupling with CFD. Particle capture criteria were discussed and used to decide the particle behavior if particles collide on the reactor wall. The slag flow model was established from this criterion to calculate the mass flow rate and the thickness of the liquid slag. The slag thickness model calculated the heat transfer at each wall location, and the coupled temperature, slag thickness, and heat transfer curves were finally obtained through iteration. The models were proved reliable by comparing the simulation with industrial results. The model predicted the slag thickness of liquid and solid slags, the wall temperature, and the heat flux through the wall. The model’s establishment further understood the slag formation, slag thickness distribution, and wall temperature distribution. It also played a guiding function in industrial operations.