Abstract: Based on the characters of the reactor in the delayed supercritical process, the physical and thermal and heat transmission model were founded, and the quasi-static method was employed. The temperature field of a single fuel element in the average channel of the reactor during the delayed supercritical process was calculated and analyzed, and the results were compared with those got from the simulation platform of the same reactor. It is shown that in the slow dynamic process, the fuel element temperature field could also be calculated exactly by the quasi-static method. Besides, the model in the paper is relatively simple and has fast calculation velocity, which is of great importance to the reactor’s dynamic analysis and some accident analysis. It is also useful to the reactor’s thermal design.