The invention discloses a device for monitoring the micro-water density of SF6 on line. The device comprises a sampling air chamber, an aeration connector, a processor, a display unit and an antifreeze auxiliary heating module, wherein the sampling air chamber, the aeration connector, the processor and the display unit form a monitoring unit; a pressure sensor, a temperature sensor and a dew point sensor are mounted in the sampling air chamber, and are connected with the signal processor; the processor is used for judging signals acquired by the pressure sensor, the temperature sensor and the dew point sensor and sending a result to the display unit for displaying. The device is adapted to an extremely cold climate condition, and has safe and reliable performance, the requirement of all-weather continuous running for 7*24 hours is met, and the temperature of a running environment can be as low as -55 DEG C.