The main failure mechanism of in-vessel corium retention through external reactor vessel cooling (IVR-ERVC) happens when the local heat flux through reactor pressure vessel (PRV) wall exceeds the critical heat flux (CHF). High local heat flux in the molten pool is usually caused by the metallic layer focusing effect due to stratification. In this paper, depending on experimental correlations, both the lump parameter method and computational fluid dynamic method are used to investigate the mechanism of focusing effect. The concentration factor varying with the height of metallic layer is studied. The results show that the lump parameter method probably overestimates the wall heat flux of metal layer.