The objective of this study is to investigate the flow structure of underwater supersonic gas jets in water flow. Supersonic gas jets submerged in a liquid flow field is experimentally studied in a water tunnel. In the experiments, a high speed camera system is used to observe the evolution of the gas jet bubble, and a dynamic pressure measurement system is used to measure the pressure fluctuation under different flow velocities simultaneously. We seek to study the mechanism of the vortex structure and the pressure fluctuation phenomenon during the gas jet evolution. The obtained results conclude that the main body formation and the pressure fluctuation of the gas jets depend heavily on the ambient flow speed. The instantaneous patterns of gas jets remarkably go upward due to the gravity effect in the still water. A shear vortex will be formed by jet-flow interaction when the ambient fluid flows. Larger vortexes are formed when the main body of the jet evolves downstream and mixes with the jet shear layer. The evolution pattern and pressure fluctuation characteristics of the gas-liquid interface are educed through a detailed analysis of the shear layer vortex structure. Backward reflection of pressure fluctuation is formed accompanying the jet bulging, necking, and back-attack. Consequently, the pressure fluctuation is transferred to the fluid at the nozzle surface and the test section. The pressure measurement system is used to confirm the pressure fluctuation phenomenon. Two measuring positions are set, i.e., pressure transducers are embedded at the nozzle surface and the test section. The pressure fluctuation with magnitude of 10 kPa is measured by the nozzle surface transducer in still water. The pressure fluctuation induced by the gas jets near the nozzle exit disappears simultaneously when the ambient fluid flows. However, the amplitude of pressure fluctuation decreases at the nozzle surface but increases at the test section with the increasing flow velocity. Power spectrum analysis is carried out and shows that the mechanical energy of the water tunnel gas jets is mainly distributed in the frequency band of 0-700 Hz. A jet induced large pressure fluctuation with a dominant frequency about 200 Hz can be captured near the nozzle surface in still water. With increasing water velocity, the dominant frequency of the unsteady pressure fluctuation decreases significantly at the nozzle surface. Conversely, the flow velocity leads to an increase in the spectral intensity of the pressure at the test section.