Abstract: Aiming at the demand of optimized geometry parameter design of CPR1000 reactor vessel insulation for in-vessel retention by external reactor vessel cooling (IVR-ERVC) under severe accident conditions, Relap5/Mod3 code calculations of ERVC uncertainty propagation, with parameter combinations acquired by Latin hypercube sampling (LHS) in possible parametric ranges as input, were carried out. The calculations, sensitivity of parametric effects on ERVC function and behavior were presented. Meanwhile, nominal values of the reactor vessel insulation design parameter of a certain type passive ERVC system of CPR1000 were preliminarily determined, based on suggested ERVC-related functional reliability criteria and related statistical analysis. And with the determined geometries, ERVC functional reliability analysis was performed, which might provide CPR1000 probabilistic safety assessment (PSA) with ERVC system reliability estimation.