When core meltdown accident occurs in PWR nuclear power plant, core melt will gather at the lower head of reactor pressure vessel (RPV) and then form a molten pool, which will threaten the integrity of reactor pressure vessel. The in-vessel retention (IVR) through external reactor vessel cooling (ERVC) technology is a performance method to alleviate core meltdown accident. Therefore, accurately predicting the heat transfer characteristics of external channel of the RPV lower head will significantly mitigate the severe core meltdown accident of nuclear power plant. By analyzing the flow heat transfer characteristics of the internal molten pool and the external flow passage involved in ERVC, the simulation calculation method of the flow heat transfer of the internal molten pool and the external flow passage of the coupled pressure vessel was established. Based on ULPU structural parameters and the input description of Energy Research, Inc. (ERI) benchmark calculations, a homogeneous flow natural circulation flow model and a two-layer melt pool heat transfer model were established. By calculating the natural circulation rate and convective heat transfer coefficient and other parameters, the feasibility of the established ERVC simulation calculation method was verified.