The invention discloses an on-line monitoring device for the micro-water content of a sulfur-hexafluoride circuit breaker.The on-line monitoring device is characterized by comprising a circuit breaker air chamber (11), an air distributing chamber (15) and a charging station duty room (10); an air inlet of a three-way part (2) is installed on an air outlet of the circuit breaker air chamber (11), a first air outlet of the three-way part (2) is connected with a second check valve (16), a second air outlet of the three-way part (2) is connected with one end of an air guide tube (3), the other end of the air guide tube (3) is connected with the upper end of a first flow guide tube (12), a vertical partition plate (13) is arranged in the air distributing chamber (15), the internal space of the air distributing chamber (15) is divided by the partition plate (13) into a first cavity (17) and a second cavity (18), and the lower end of the first flow guide tube (12) is located at the bottom of the first cavity (17).By means of the on-line monitoring device, humidity signals of sulfur hexafluoride are converted into capacitance signals to be transmitted to a control device through a humidity sensor, the control device converts analog signals into digital signals and transmits the digital signals to the charging station duty room, and an operator on duty can monitor humidity information in real time accordingly.