Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) plays an important role in removing the core decay heat under non-Loss of Coolant Accident (LOCA) accident. In the initial stage of the Passive Residual Heat Removal System (PRHRS) activating, the natural convection is the main way to exchange heat between the secondary side of the PRHR HX with IRWST. However, it is almost impossible to acquire the thermohydraulic performance in IRWST since the influence of the natural convection and thermal stratification in IRWST happens simultaneously. In this paper, Hierarchical Two-Tiered Scaling (H2TS) method is employed to get scaling criteria based on control equations. Anda visualized water tank with five C-shape electrical heating tubes was scaled-down and applied to investigate the thermohydraulic performance for IRWST of CAP1400 Nuclear Power Plants (NPPs). Particle Image Velocimetry (PIV) measurement was used to investigate the evolution of the flow field of total twenty-five surfaces in three axial directions under three steady- and two variable-heating conditions. Meanwhile, three thermocouple bundles were adopted to acquire temperature data in three regions. The experimental results show that the temperature deviation in the same height of the IRWST is flattened by plenty of vortexes. As a result, local circulation and the temperature difference is less than 1 degrees C. While the thermal stratification along the vertical direction is obvious, the distribution of flow field and temperature demonstrate that a "dead zone" exists in bottom of the IRWST. Besides, a "thermal interface" region forms in the lower middle region as hot and cold fluid mixing. An obvious upwelling flow is observed near the PRHR HX heating rod bundle from flow fields of three axial directions. At the same time, the maximum height of the upwelling flow decreases with time increasing. On the one hand, the natural convection is induced by the thermal stratification. What is more, the thermal stratification will impair the scale and strength of natural convection, which will intensify the thermal stratification conversely. (C) 2019 Elsevier Ltd. All rights reserved.