Abstract: Using MELCOR code, the different models of molten material stratification in reactor cavity for small power reactor blackout severe accident after pressure vessel lower head failure were calculated, and the effect of molten material stratification on accident consequence was researched. The results show that the total thickness and the change trend of molten material in different models are basically consistent. When choosing concrete as the base material of reactor cavity, the stratification of core molten material is relatively complex, and choosing metal material as the base material of reactor cavity, the stratification of core molten material is relatively simple. The molten core-concrete interactions （MCCI） will not occur for small power reactor. The calculation of the different models has no influence on radioactive consequence, but has some influence on melting process of the reactor cavity. The melting time of enforced mixing model is the fastest, and the melting time of mechanistic calculation model is the slowest. For safety analysis, choosing enforced mixing model is more conservative.