Abstract: The periodic flooding characteristic of annular flow in a narrow rectangular duct is one of the phenomena of air and liquid flowing in reverse through rod bundle structure space of the reactor core, and the research on the characteristics of flooding behaviors, occurrence conditions, and flooding mechanism can provide data and theoretical support for the accident treatment of nuclear power plant. In this study, the periodic flooding characteristics of annular flow in a narrow rectangular duct were investigated, the high-speed photographic observation shows that each cycle of the flooding process consists of four flow regions, i.e., the onset flooding region, the full flooding region, the liquid membrane reversal region and the liquid phase downstream region. When the liquid phase flow rate is low, the annular flow flooding period increases slightly with the increase of the gas phase velocity, and when the liquid phase flow rate is high, the flooding period decreases with the increase of the gas phase velocity. The time proportion of the full flooding region gradually increases with the increase of the gas phase velocity, and the time proportion of the liquid phase downstream region gradually decreases until the annular flow reaches the complete rising pattern. In the area of low liquid phase velocity, the time of the onset flooding region increases with the increase of the gas phase velocity, while the time of the liquid membrane reversal region remains the same. In the area of high liquid phase velocity, the time of the onset flooding region and the liquid membrane reversal region decreases correspondingly. According to the experimental results, the mechanism of the periodic flooding behavior analysis was obtained. The thickness of the liquid film in annular flow increases because of the space bondage in the narrow rectangular duct, which makes the periodic flooding time increasing and the flow range of downward liquid phase increasing at the same time. The fundamental reason for the periodic flooding of annular flow in the narrow rectangular duct is the “protection” effect of the channel structure on the top layer of the liquid phase. The multiscale permutation entropy of the annular flow is disordered when the gas phase velocity is small, while the entropy of the annular flow gradually decreases with the increase of the gas phase velocity when the gas phase velocity is high, so the judgment conditions of the liquid phase declining and complete rising can be determined by the conversion point of “unordered” to “ordered” of the multiscale permutation entropy.