The invention provides a water injection sealing body which comprises a movable seal composed of a sealing ring, a static seal comprising a sealing cavity, and a pressing device used for pressing or loosening the static seal. The movable seal, the static seal and the pressing device are sequentially and adjacently arranged and are arranged in a cavity, wherein the cavity is provided with two gas cavity connectors, the first gas cavity connector 18 communicates with the static seal, and the second gas cavity connector 19 communicates with the pressing device. The invention further provides a method for online replacing a nozzle of a compressor water injection system. According to the water injection sealing body and the method, compressed gas in the compressor cannot be leaked and is not polluted when the nozzle is replaced online, and a very good sealing effect is achieved.