A monitoring method for the soil gas multicomponent cumulative release amount of a CO2 oil displacement area calculates the soil gas amount released from the CO2 displacement area by using a method of replacing saturated brine or water with the soil gas , and detects the content of each component in the soil gas by using a gas component analyzer, comprising: covering the soil surface of the CO2 oil displacement area with a sampling cage, connecting the air outlet of the sampling cage with the inlet of a graduated vessel through a flexible pipe, and with the inlet of the gas component analyzer through a tee-junction, connecting the outlet of the gas component analyzer with the inlet of the sampling cage, firstly feeding the soil gas into the vessel for replacing the saturated brine or water in the vessel, feeding the saturated brine or water into another open-mouthed vessel through a glass tube and a flexible tube, calculating the cumulative gas amount of released soil gas through the dropped graduation caused by the saturated brine or water on the vessel, and detecting the content of each component in the soil gas, or directly on line detecting the momentary release amount and the content of each component of the soil gas through the gas component analyzer.