The invention discloses gas online monitoring system. The gas online monitoring system comprises a sampling and recharging gas path and a circular detection gas path, wherein the circular detection gas path is an annular closed gas path; a first two-way electromagnetic ball valve, a second two-way electromagnetic ball valve, a flowmeter, a small flow gas pump, a measuring instrument, a first pressure transmitter, a buffer tank and a large flow gas pump; the flowmeter is a flowmeter capable of adjusting the flow; a sulfur hexafluoride gas source is connected to a gas inlet of the sampling and recharging gas path; a gas outlet of the sampling and recharging gas path is connected between the first two-way electromagnetic ball valve and the second two-way electromagnetic ball valve of the circular detection gas path. The invention also discloses a monitoring method of the gas online monitoring system. According to the gas online monitoring system and the monitoring method disclosed by theinvention, by using an online monitoring mode of adopting high-voltage switch equipment for sampling, enabling sampling gas to circulate in a detection gas path and finally pressing the sampling gas into the high-voltage switch equipment, the defects of an existing online on-voltage measurement mode that the sampling gas has no representativeness and measurement data reliability is difficult to ensure can be avoided.