Abstract: In-vessel retention (IVR) of molten material, as one of the most important mitigation measures for serious accidents, can effectively prevent the leakage of molten material from the core to the outside of the pressure vessel, resulting in the release of a large amount of radioactive material. Hua-long Pressurized Reactor (HPR1000) has set up a combined active and passive reactor cavity injection cooling (CIS) system to cope with similar extremely serious accidents. Under severe accident conditions, it implements external pressure vessel cooling to achieve the IVR of molten material. To ensure the success of the molten material retention strategy, it is necessary to maintain the heat flux on the outer wall of the pressure vessel lower head below the critical heat flux (CHF) under this operating condition during the implementation of external reactor vessels cooling. In order to evaluate the external pressure vessel cooling capacity limit of the active subsystem of the CIS system, a rectangular heating element with equal width was used to simulate the lower head of the pressure vessel. Under forced driving cooling conditions, the influences of different operating parameters such as inlet subcooling, cooling flow rate, and pressure on the critical heat flux were experimentally studied. The gap spacing size, outlet pressure, inlet subcooling and other conditions of the sealed flow channel of the experimental simulation are consistent with the external cooling process of the lower head of the CIS system of HPR1000. In addition, the wall heat flux and coolant flow rate of the CIS system were also accurately simulated by means of a specific modeling method. A discrete plurality of electric heating elements was used as heat sources, and the power of each group of electric heating elements was controlled independently in groups to achieve the desired heat flux distribution. The experimental results indicate that, under the conditions of 0.1 MPa at the outlet pressure of the pipeline and 400 t/h and 860 t/h coolant flow rate of the CIS system, the critical heat flux decreases with the increase of angle in the area near the outlet. In the central region, the critical heat flux also increases with the angle. The experimental results also indicate that, in the area near the inlet, the critical heat flux decreases with the increase of angle, and the effect of the inlet effect is more significant when the subcooling at the inlet is high.