Steam-seawater jet can be one of the main solutions for reactor pressure control after an accident in a marine nuclear power plant. However, there is a lack of research on the phenomenon of steam immersion jet under seawater conditions, and the pressure oscillation characteristics of steam low-velocity immersion jet under seawater conditions are still unclear. Through experimental research, the differences between the typical dynamic pressure signal, pressure oscillation intensity, pressure oscillation amplitude and pressure oscillation frequency of low-velocity steam jet condensation in seawater and low-velocity steam jet condensation in freshwater are compared and analyzed. The results show that there are differences between the steam-seawater jet and the steam-freshwater jet, especially the pressure oscillation amplitude in the pool, and the pressure oscillation amplitude in the steam-seawater jet is much smaller than that of the steam-freshwater jet under higher water temperature. The research in this study is helpful to understand and grasp the pressure oscillation characteristics of steam-seawater jet.