Abstract: With air-water as working fluid, the tests of flooding in AP1000 surge line are conducted at atmospheric pressure and room temperature with different dimensional water levels in the pressurizer simulator. Mechanism analysis is performed on the flooding phenomena and data, and their dynamic nature has been captured. The research result shows that, flooding is most likely to transpire on the vertical part of the surge line, flooding characteristic coincides with the Kutateladze relation, the water flow rate decreases as the pressurizer simulator water level increasing in small gas flow conditions, the water flow rate increases as the pressurizer simulator water level increasing in large gas flow conditions, and zero water penetration point is almost independent of the pressurizer simulator water level.