As an important phase of a loss of coolant accident, the reflooding process can effectively mitigate the severity of the accident, and its flow pattern evolution behavior can well reveal the reflooding process. To study the evolution of liquid phase during the re submergence process of rectangular narrow channels, bottom reflooding experiments were conducted under different thermal parameters (inlet velocity and inlet subcooling). Systematically explained the evolution process of full flow pattern during reflooding. Based on full flow-pattern visualization data, image processing steps such as background elimination, erosion and dilation were carried out to obtain the temporal variation law of liquid fraction. The differences in liquid fraction under different working conditions were analyzed, and the spectrum relationship of liquid fraction under each working condition was obtained through filtering and Fourier transform. The intermittent flooding characteristics of the reflooding process were elucidated. The results showed that through the time-domain image of liquid phase fraction, it was found that the reflooding process only had a brief droplet flow stage and would enter the invert-annular flow heat transfer stage faster; Through Fourier transform, it was found that during the reflooding process, the main frequency of the liquid phase fraction change was concentrated in the low-frequency range of 1–5 Hz.