The invention discloses a sulfur hexafluoride (SF6) gas micro water on-line monitoring device of a high-voltage electrical device, a method and a system, wherein the high-voltage electrical device SF6 gas micro water on-line monitoring device comprises a computer data processing system, a gas circulation mechanism and a micro water detector which detects gas in the gas circulation mechanism, wherein the gas circulation mechanism comprises an air cylinder, a piston and a driving device which enables the piston to move back and forth in the air cylinder, the air cylinder is provided with a circulation air vent which is sealed and communicated with a sulfur hexafluoride gas storage chamber which is arranged in a high-voltage electrical device, and the gas circulation mechanism is arranged in a sealed cavity. Gas in the sulfur hexafluoride gas storage chamber arranged in the high-voltage electrical device is circulated to the air cylinder which is arranged outside the high-voltage electrical device through the sealed gas circulation mechanism, thereby enabling humidity of sampling points inside and outside the sulfur hexafluoride gas storage chamber to be capable of rapidly achieving balance, not needing to discharge a large number of SF6 gas by adopting the micro water detector to accurately detect, and being accurate in detection result and safe in performance.