Abstract: To clarify the effects of the cross-sectional sizes and water depths on the gas-liquid countercurrent flow characteristics (CCFC) in horizontal rectangular pipes, a visualization CCFC experiment was carried out through an underwater exhaust experimental system using air and water as the gas and liquid phase. In the experiment, a rectangular plexiglass pipe in a length of 2 m and inner cross-section scale of 106×60 mm was tested under the transversal and upright layout, respectively, and each layout was tested under the water depth of 1 m and 3 m. Conclusions can be drawn as follows: (1) The height of flow channel has a significant effect on the horizontal CCFC in pipes. The liquid phase backflow can be greatly enhanced if using a higher channel height under a given cross-sectional area of pipes, which is beneficial to condensate water reflux in hot legs thus ensuring the cooling of reactor cores under the loss of coolant accident. (2) The dimensionless CCFC curves of the transversal and upright layout, which curves are expressed by the Wallis parameters of the gas and liquid phase, were coincided when the channel height was used as the characteristic scale of Wallis parameters, meaning that the CCFC of the horizontal pipes in different cross-sectional sizes could be well correlated using such a definition of Wallis parameters. (3) The influence of changing water depth on the dimensionless CCFC in horizontal rectangular pipes is insignificant in the range of water depth 1-3 m. Finally, an experimental correlation was proposed for predicting the CCFC in horizontal rectangular pipes.