A CCUS system for exploiting a thickened oil reservoir based on an optimal flue gas CO 2 enrichment ratio, characterized by comprising a flue gas CO 2 enrichment unit, a flue gas injection unit, a thickened oil thermal production well group unit and a produced gas recovery unit; the fuel gas CO 2 enrichment unit comprises an air separating enrichment unit and a boiler injection gas premixed tank; the air separating enrichment unit comprises an air separating primary device used for separating air into oxygen and nitrogen preliminarily, wherein the air separating primary device comprises an air film separating primary device, a nitrogen-rich conveying pipeline (b), a first oxygen-rich conveying pipeline (a); the nitrogen-rich air conveying pipeline is connected with the nitrogen-rich gas pressurization monitoring tank; and the first oxygen-rich air conveying pipeline is connected with the oxygen-rich gas pressurization monitoring tank through a second oxygen-rich air conveying pipeline (c); and an air separating secondary device used for further enriching a part of the oxygen which is subjected to the preliminary separation, wherein the air separating secondary device comprises an air film separating secondary device; and the first oxygen-rich air conveying pipeline is connected with the oxygen-rich gas pressurization monitoring tank through a third oxygen-rich air conveying pipeline (d), the air film separating secondary device, and a secondary oxygen-rich air conveying pipeline (e); and the boiler injection gas premixed tank is used for mixing the preliminarily separated nitrogen, the preliminarily separated part of the oxygen and the further enriched oxygen; wherein the flue gas injection unit comprises a boiler connected with the boiler injection gas premixed tank through a boiler injection gas pressure stabilizer; the flue gas outlet of the boiler is connected with the flue gas monitoring tank through the boiler flue gas conveying pipeline, and the flue gas monitoring tank is connected with the boiler injection gas premixed tank; the thickened oil thermal production well group unit comprises a thermal production well group; the flue gas monitoring tank is connected with the thermal production well group through the thermal production well mouth injection device; and the boiler is connected with the thermal production well mouth injection device through a steam conveying pipeline (h); and the produced gas recovery unit comprises a gas-liquid separation device connected with a thermal production well group; the gas-liquid separation device is also connected with a produced gas pressurization monitoring tank through a produced gas conveying pipeline; and the produced gas pressurization monitoring tank is connected with the boiler injection gas premixed tank. the fuel gas CO 2 enrichment unit comprises an air separating enrichment unit and a boiler injection gas premixed tank; the air separating enrichment unit comprises an air separating primary device used for separating air into oxygen and nitrogen preliminarily, wherein the air separating primary device comprises an air film separating primary device, a nitrogen-rich conveying pipeline (b), a first oxygen-rich conveying pipeline (a); the nitrogen-rich air conveying pipeline is connected with the nitrogen-rich gas pressurization monitoring tank; and the first oxygen-rich air conveying pipeline is connected with the oxygen-rich gas pressurization monitoring tank through a second oxygen-rich air conveying pipeline (c); and an air separating secondary device used for further enriching a part of the oxygen which is subjected to the preliminary separation, wherein the air separating secondary device comprises an air film separating secondary device; and the first oxygen-rich air conveying pipeline is connected with the oxygen-rich gas pressurization monitoring tank through a third oxygen-rich air conveying pipeline (d), the air film separating secondary device, and a secondary oxygen-rich air conveying pipeline (e); and the boiler injection gas premixed tank is used for mixing the preliminarily separated nitrogen, the preliminarily separated part of the oxygen and the further enriched oxygen; wherein the flue gas injection unit comprises a boiler connected with the boiler injection gas premixed tank through a boiler injection gas pressure stabilizer; the flue gas outlet of the boiler is connected with the flue gas monitoring tank through the boiler flue gas conveying pipeline, and the flue gas monitoring tank is connected with the boiler injection gas premixed tank; the thickened oil thermal production well group unit comprises a thermal production well group; the flue gas monitoring tank is connected with the thermal production well group through the thermal production well mouth injection device; and the boiler is connected with the thermal production well mouth injection device through a steam conveying pipeline (h); and the produced gas recovery unit comprises a gas-liquid separation device connected with a thermal production well group; the gas-liquid separation device is also connected with a produced gas pressurization monitoring tank through a produced gas conveying pipeline; and the produced gas pressurization monitoring tank is connected with the boiler injection gas premixed tank.