The invention discloses an on-line monitoring system for oil gas recovery of a gas station, which comprises a flowmeter, a vacuum pump, a vacuum/pressure transmitter and a data acquisition module, wherein a Roots type flowmeter is connected in serial on a gas returning pipeline in front of the vacuum pump and at the back of a fuel truck nozzle in an oiling machine and is used for measuring the extraction flow of the vacuum pump; and the vacuum/pressure transmitter is in by-pass connection with a main gas returning pipe through a three-way pipe and is used for measuring the pressure at the main gas returning pipe and in a buried pipe. The data acquisition module further comprises one or more microcontrollers; and the data acquisition module is used for counting pulse output signals of the Roots type flowmeter and timing at the same time. The on-line monitoring system for the oil gas recovery of the gas station judges whether the gas liquid ratio, the air tightness and the pipeline hydraulic resistance of an oil gas recovery system are normal through continuous gas liquid ratio data and pressure data, can give an alarm to remind operating staff to take a corresponding measure when an abnormality is found, and can record, store, process and transmit data for a supervision department to examine.