Abstract: The passive heat removal system is one of the important devices to ensure the residual heat discharge of the nuclear power plant under the accident. When the core residual heat is discharged, the steam in the horizontal pipeline condenses due to the direct contact with the low-temperature seawater, resulting in the instability of the gas-liquid interface and even the condensation-induced water hammer (CIWH). This study aims to simulate the formation and evolution process of CIWH in the horizontal pipeline and study the transient interface features by visualization techniques. A visual experimental device was constructed to measure the pressure pulse in the pipeline. The visual image data of the gas-liquid interface was collected, and the similarities and differences between the evolution process of the two-phase flow interface instability and the CIWH were compared. Using each column in the spatial image data set, the temporal image of a specific position was generated, realizing the mutual verification of the pressure data and the timing image. The effects of different water temperatures and different heating powers on CIWH were investigated, and the distribution probability of the CIWH pressures and the gas-liquid interface fluctuations in time-series images were compared under different working conditions. The two-phase interface in the time-series images was extracted and the interface evolution process was analyzed. Direct contact condensation (DCC) in the horizontal tube will fluctuate the gas-liquid phase interface and induce instability. When the condensation is relatively mild, the interface will quickly restore the stable laminar flow state; If the condensation is severe, the interface instability will further evolve into the plug flow and eventually lead to the CIWH phenomenon. CIWH has obvious periodic characteristics. The pressure data can correspond well with the time-series images of the two-phase flow, which can capture the occurrence of CIWH. The time-series images can be used as supplement for determining the onset of CIWH along with the pressure signal. The period of CIWH reduces with the increase of the heating power, and the CIWH under a high heating power occurs more frequent. Increasing the temperature of the cold water can inhibit the occurrence of CIWH. There is no obvious fluctuation in the gas-liquid interface in the time-series images, and the CIWH no longer occurs. The heating power and cold water temperature has a large effect on the pressure distribution probability of CIWH. Increasing the heating power strengthens the fluctuations of the gas-liquid interface according to the time-series images.