A device and method for injection of oxygen-rich gas into a body of liquid with oxygen recycling are disclosed. The device comprises a rotary hollow shaft vertically passing through a float partially immersed in the liquid, an impeller attached to the lower end of the rotary hollow shaft, a columnar structure, surrounding the rotary hollow shaft, mounted on the bottom side of the float and vertically extending into the liquid, a gas diffusion chamber formed by the columnar structure, the float and the liquid surface under the float, and a gas injection conduit passing through the float for delivering the oxygen gas into the gas diffusion chamber, wherein a vacuum is generated in the body of the liquid around the impeller when the impeller is driven to rotate, so that the oxygen-rich gas in the gas diffusion chamber is sucked into the body of the liquid and mixed therein.