Abstract: To address the application demand of replacing freshwater with seawater as the cooling medium in the suppression pool of marine nuclear power plants such as ships, an experimental study on the condensation flow patterns of steam-seawater jet was conducted. A visual experimental setup was established, and high-speed photography and dynamic pressure measurements were employed to compare the flow pattern evolution and pressure oscillation characteristics of steam-seawater jet and steam-freshwater jet under low mass flux conditions. The results show that steam-seawater jet and steam-freshwater jet exhibit highly consistent flow pattern distributions under the same nozzle diameter, with the flow pattern transitioning from chugging to condensation oscillation and then to bubble condensation as water temperature increases. The chugging flow pattern occurs under low water temperature and low mass flux conditions, where the dynamic pressure signals of the two jets are identical: Pressure fluctuations in the water pool appear as weak oscillations accompanied by spikes, while pressure oscillations inside the nozzle present as sinusoidal waves with amplitudes reaching hundreds of kilopascals. Under the condensation oscillation flow pattern, the steam-seawater jet and the steam-freshwater jet both exhibit the “necking” phenomenon, and the oscillation period increases with rising water temperature but decreases with increasing steam mass flux. However, in the seawater jet, the small bubbles generated after bubble collapse are smaller in size and larger in number, tending to accumulate in the pool and surround the main bubble, which can interfere with clear observation of the phase interface. Regarding dynamic pressure signals, the pressure fluctuations inside the nozzle are similar for the steam-seawater jet and the steam-freshwater jet, but differences are observed in the water pool: At a water temperature of 40 ℃, the peak pressure in the seawater pool is slightly higher than that in the freshwater pool; At 65 ℃, due to the dense layer of small bubbles that weakens pressure propagation, the peak pressure in the seawater pool is significantly lower. The bubble condensation flow pattern appears when the water temperature rises above approximately 80 °C. At high temperatures, the large bubble remains stably attached to the nozzle and slowly breaks up along the nozzle wall, with the generation rate of small bubbles lower than their dissipation rate, allowing the phase interface to become clearly visible again under seawater conditions. At this stage, the steam-water interface in the steam-seawater jet and the steam-freshwater jet exhibits only slight fluctuations, with pressure oscillation amplitudes within ±5 kPa. The findings reveal the flow pattern boundaries and evolution characteristics of seawater jets, identify the key differences between the steam-seawater jet and the steam-freshwater jet under the condensation oscillation flow pattern and their underlying causes, and provide fundamental data and theoretical support for the design and safety analysis of using seawater as a substitute for freshwater in suppression pool cooling systems.