To avoiding the leakage of radioactive materials after the reactor core melting accident, the third-generation million-kilowatt nuclear power units in China have adopted the external cooling of the reactor pressure vessel (RPV) to implement the melt retention strategy (IVR-ERVC). The key criterion for the success of IVR-ERVC strategy is ensuring that critical heat flux (CHF) does not occur on the heat exchange surface during the cooling process, which could lead to RPV rupture. This study focuses on the HPR1000 reactor as a prototype and constructs a 1/5 scale-down 3-D downward-facing boiling test facility, REVECT-3D. The facility primarily consists of an RPV simulator, a flow channel, and a cooling water system. The RPV simulator has a diameter of 1 m, and the flow channel structure replicates the prototype. To simulate the heat flux distribution of the molten pool in the prototype reactor, the lower head of the REVECT-3D RPV simulator is divided into eight independent heating zones, with a maximum heating power of 1.8 MW and a maximum heat flux of approximately 2.0 MW/m2. This paper investigates the influence of circulating water velocity on CHF in the low azimuth region of the RPV simulator. Experimental results indicate that as the cooling water flow rate increases from 20 m3/h to 60 m3/h, the CHF at the bottom of the RPV increases from approximately 0.8 MW/m2 to 1.2 MW/m2.