The IVR-ERVC system achieves the retention of molten material inside the reactor through external cooling of the pressure vessel, which is one of the most effective severe accident mitigation measures to ensure the integrity of the pressure vessel. To ensure the effectiveness of passive ERVC measures, it is necessary to conduct research on the two-phase characteristics and CHF in the ERVC flow channel. For the IVR system of the CAP1400 reactor model, a passive full height IVR test device with a height ratio of 1:1/cross-sectional area ratio of 1:100 was designed and constructed based on the design parameters of the pressure vessel, insulation layer, reactor cavity, and submerged structure of the reactor cavity. Experimental research on the two-phase distribution characteristics in the ERVC flow channel was conducted. The research results indicate that the natural circulation flow rate of ERVC increases overall with the increase of total heating power, but after reaching a certain flow rate, the growth rate slows down; Bubbles in the flow channel are mainly generated and developed along the heated wall. As the heat flow increases, the flow pattern gradually transitions from bubbly flow to slug flow to turbulent flow. When CHF occurs, the wall is almost completely covered by atmospheric bubbles; The average void fraction near the wall is mainly determined by the local heat flux, and the radial void fraction distribution shows a pattern of decreasing as the distance from the heated wall increases; The distribution shape of circumferential void fraction is relatively similar to that of heating heat flux distribution. During the research process, real prototype conditions such as full height modeling, non-uniform heating, and wall heating of pressure vessel prototype materials were used. The natural circulation two-phase distribution characteristics obtained from the experiment can be directly used for the development of passive IVR-CHF mechanism models.