Abstract: For the special structure of the fission blanket, the heat conduction model in the core package of the MELCOR code is modified. Based on the new code, an analysis model is developed for the fission blanket and the cooling loop. After verification of the modeling method, the performance of the Subcritical Energy Reactor（SER） under the station blackout accident（SBO） is simulated. Simulation result shows that: the higher the power density in the fission blanket, the earlier being exposed, and the quicker the meltdown will be. The fuel at the upper part of the inner blanket begins to melt firstly, and then the fuel of the outer blanket, where the melt both occur more than two hours after being exposed. The results also show that, the zirconium water reaction affects significantly the accident process. In some local fission areas, the heat from the chemical reaction becomes the main heat source, which leads to the fuel temperature rising, or even the melt.