Abstract: The Evolution and dynamic behavior of bubble underwater is an important theory basis for the gas-liquid two-phase flow and application of underwater jet. The forming bubble and gas jet through annular nozzle have di erent performances and mechanism with bubble and jet generate through round hole. With the requirement of special application such as the concentric cylinder water piercing missile launching, observing the basic phenomena and mechanism analysis of the annular nozzle gas jet / bubble flow become an urgent need. A set of experimental system of the annular nozzle were designed and the evolution of bubbles through the annular nozzle was investigated. The visualization of bubble flow was realized by a high speed camera, which was used to record the bubble growth process. The bubble formation regimes, the influence of gas flow rate on bubble formation periods and bubble volume were studied based on the image processing and analysis of flow field.The experimental results indicate that: the regime of bubble formation is a period-3 bubbling under air flow rate from 50.8 dm³/min to 237.3 dm³/min; The variation of bubble formation periods and bubble volume with gas flow rate was given, which indicates that bubble formation periods and bubble volume are strongly dependent on the gas flow rate. Because of the cooperation of the surface tension, water inertia and liquid flow, top of the bubble will collapse downward during formation. It will lead to form an toroidal bubble.