Ultra-supercritical coal-fired power generation technology is an advanced and efficient power generation technology. Due to the high parameters of the ultra-supercritical boiler, the heat transfer of the working substance in the water wall is complicated. Therefore, this paper takes an ultra-supercritical boiler as the research object and uses MATLAB to calculate the temperature of working substance inside the water cooling wall. The results show that when operating under low load there is a phase change zone in the working substance of the water cooling wall, the heat transfer is complex in this zone, and the temperature of the water cooling wall may rise to dangerous range. Therefore, it is necessary to pay attention to the fluctuation of temperature of working substance.