Abstract: For low permeability volatile reservoirs,elastic production can easily become depleted due to its special fluid properties,therefore,gas injection and pressure-bearing is the main production method.For the technical difficulties in the gas injection,the injection timing of CO_2 flooding in low permeability volatile reservoirs was studied by long core experiments.The results show that when the formation pressure is depleted to 34.10 MPa,the degree of CO_2 production(96.44%)is equivalent to the degree of CO_2 production(96.24%)under the original formation pressure.The degree of CO_2 production is significantly reduced after the formation pressure is depleted to 28.58 Mpa.It is concluded that when the formation pressure is depleted to 34.10 Mpa,it is the timing of the injection of CO_2 flooding.The research results provide an experimental basis for the economic and effective implementation of CO_2 flooding to improve oil recovery in low permeability volatile reservoirs.