The invention provides a method for online accurate humidity measurement of sulfur hexafluoride gas in a GIS. SF6 gas in the GIS is automatically sampled and measured according to a specific measurement time interval requirement, and a result is transmitted to a monitoring background; when the numerical value of the measured SF6 gas exceeds standards, alarming is conducted in time, and operation and maintenance staff are prompted to find out causes; when humidity testing of the SF6 gas in equipment is qualified, the system can immediately start an inflation pressure pump, an inflation valve is turned on, the GIS equipment is filled with gas in an SF6 gas recycling device, and it is guaranteed that the gas is measured in a flowing state. Devices used in the method include the GIS equipment (1) filled with the SF6 gas, a sulfur hexafluoride gas pressure meter (2), a sulfur hexafluoride gas humidity sensor gas obtaining valve (3), a sulfur hexafluoride gas humidity sensor (4), a sulfur hexafluoride gas humidity online measurement background (5), the sulfur hexafluoride gas recycling device (6), a pressure pump (7) and an adjustable gas supplement valve (8).