Abstract: In order to address the research gap in steam-seawater interactions and evaluate the feasibility of using seawater as a coolant in marine nuclear power systems, the pressure oscillation characteristics of steam low-velocity jet in seawater have been investigated. The motivation stems from the fact that seawater, covering 70% of the Earth’s surface, offers practical advantages over freshwater for long-term maritime operations, yet its impact on steam jet dynamics remains poorly understood. The experiments focus on comparing pressure oscillations in seawater and freshwater under varying steam mass flow rates (20, 40, 60 kg/(m²·s)) and water temperatures (30-90 ℃), with the goal of assessing system safety and performance. The experimental setup consisted of a steam supply system, a stainless-steel water tank, and instrumented nozzles equipped with high-frequency dynamic pressure sensors (sampling rate: 20 kHz). Steam was injected into seawater or freshwater-filled tanks, and pressure oscillations in both the nozzle and tank were measured. Time-domain and frequency-domain analyses, including fast Fourier transform (FFT), were applied to characterize oscillation intensity and frequency. Repeatability tests confirmed data reliability. Key findings reveal that steam-seawater jets exhibit pressure oscillation behaviors similar to freshwater jets but with notable differences. At low steam mass flow rate (20 kg/(m²·s)) and temperatures below 60 ℃, seawater jets produce stronger oscillations due to higher density, which reduce pressure wave attenuation. In contrast, at higher mass flow rates (40-60 kg/(m²·s)), nozzle pressure oscillations converge between seawater and freshwater, while tank oscillations in seawater show nonlinear trends—initially stronger but weaker at elevated temperatures. This shift is attributed to smaller, denser bubbles in seawater, which enhance pressure wave scattering. Frequency analyses show that both systems share similar dominant frequencies (10-200 Hz), decreasing with rising temperature due to slower bubble collapse. The study concludes that seawater can alter pressure oscillation intensity but does not significantly affect frequency patterns. For marine applications, seawater’s higher density may amplify low-flux oscillations, but its bubble-suppressing properties could mitigate high-temperature effects. These insights advance the understanding of steam-seawater jet dynamics and support safer design options for marine nuclear cooling systems.