The invention provides a compressor water injection system capable of replacing a nozzle on line. The compressor water injection system comprises a water supply system 1, a spray head 2 comprising the nozzle, a water injection sealing body 3 arranged in a cavity, a ball valve 4, a nitrogen charging device 5 and an exhaust gas collecting device 6; the water supply system 1 is connected with the spray head 2; the spray head 2 sequentially passes through the water injection sealing body 3, and the ball valve 4 is connected to the inside of a compressor; and two air cavity interfaces are arranged on the cavity. The invention also provides a method for replacing the spray head based on the compressor water injection system capable of replacing the nozzle on line. The spray head can be drawn out of a compressor runner under the condition that the compressor operates without halt, and after the nozzle is replaced, the spray head is remounted in the compressor, so that the water injection and spray operations are continued. In addition, the compressed gases in the compressor are not leaked and polluted when the nozzle is replaced on line.