The invention discloses an external cooling test system and method for a pressure vessel and belongs to the technical field of external cooling of the pressure vessel. The test system comprises a testpiece, a high-temperature molten pool, a cooling water supply loop, a steam cooling loop, a nitrogen source as well as a corresponding power distribution cabinet, instrument control equipment and data collection equipment. The test method comprises steps as follows: metal in the high-temperature molten pool is molten into a liquid state, and molten metal is delivered to the test piece under nitrogen pressure; the test piece adopts an impeller to drive electrically heated liquid metal to circularly flow in the test piece, the liquid metal is utilized to simulate in-reactor melt under a seriousaccident, heat is transmitted to a lower sealing head of the test piece, and the outer surface of the lower sealing head soaked in cooling water is boiled to realize heat exchange; the cooling waterrequired by the test is guaranteed by the cooling water supply loop and the steam cooling loop. According to the system and the method, the work of studying influence of the outside structure surfaceand a flow channel structure of the lower sealing head on external cooling performance of the pressure vessel can be carried out, thereby being beneficial to improvement ofsafety margin of external cooling of the pressure vessel.