Abstract: A molten pool comes into being after core damage under severe accident conditions in PWR plant. It has many uncertainty factors about the forming process of the molten pool, and this will affect the validity of in-vessel retention (IVR) strategy of reactor vessel. Based on the two-layer IVR model of AP1000, the study on the parameters uncertainty of molten pool under assumed severe accident conditions for external reactor vessel cooling (ERVC) was achieved, including sensitivity analysis and Latin hypercube sampling (LHS) probability analysis of the parameters. The results show that decay power has the essential effect on IVR evaluation parameters. Actions such as water injection from upper plenum should be taken to delay the core molten time. The mass of stainless steel in debris has great impact on parameters of molten metal layer. Some penalty materials could be placed in vessel to mitigate the heat focus effect of the metal layer. The invalidation probability of vessel outside of molten ceramic layer is merely 1.2%, but the invalidation probability of vessel outside of molten metal layer reaches up to 20%. This study has certain directive significance on future IVR strategy research and design, and also provides theoretical support to safety evaluation of PWR plants.