Abstract: Bubbling deaeration is widely applied in marine steam power system (MSPS), and direct contact condensation (DCC) of steam injected into water under low sub-cooled water is the thermodynamic foundation of this technology. The deaeration process involves complex thermal and hydraulic behaviors, such as violent turbulence, interfacial mass transfer, and multiphase flow etc., which induce significant pressure fluctuation and radiated noise. And it is an important element affecting the acoustic stealthiest of marine. In this paper, the two-phase flow regimes and acoustic characteristics of DCC of steam injected into water were studied for sub-cooled temperature ranging from 2.8 ℃ to 14.7 ℃. It is found that both the steam mass flux and the sub-cooled temperature have significant influences on steam plume behaviors. Wide-band noise was observed during the low sub-cooled DCC process. Low-frequency component occupies the dominant position of radiated noise at very low sub-cooled temperature conditions. The contribution of high-frequency noise increases gradually with sub-cooled temperature.