Abstract: During the starting process of coal boiler, the speed of feed water should be limited to avoid thermal plastic deformation. The key for better understanding of this mechanism lies on calculation of the temperature difference between the top and bottom part of the vapor bag. During the starting process, the two phase flowing state inside the vapor bag changes continuously, making the related calculation of temperature distribution will be more complicated. Experimental and extrapolating method of math has been used to get the strength of thermal source inside the vapor bag. Limited element method has been used to solve the related thermal transferring problem, which got useful results.