Abstract: Taking the in-containment refueling water storage tank (IRWST), which is widely used in generation Ⅲ nuclear power technology, as the object of study, the similarity criteria for natural convection phenomena were obtained by scaling analysis in terms of normalized control equations. The scaled down facility was designed, the natural convection phenomena in IRWST under accident conditions were experimentally studied and the natural convection evolution process and the influences of initial condition on it were analyzed. The results show that modified Grashof number, modified Reynolds number and modified Prandtl number are the similarity criteria for design of test facility on natural convection in IRWST. In the initial heating stage, the axial rising plume is the dominant flow in IRWST. With the formation of thermal and cold stratification, the upward motion of fluid is suppressed, and the upward motion is transformed to the radial cross flow in the middle and lower regions of IRWST. The evolution of natural convection in IRWST is basically the same under different initial conditions, but the evolution rate of flow field and fluid velocity are different.