A composite cooling system, comprising an indoor air duct and an outdoor air duct that are independent of each other, wherein the indoor air duct and the outdoor air duct intersect in a heat exchange area of the composite cooling system; a first-stage heat exchanger core, a second-stage heat exchanger core, and a first side air duct are disposed in the heat exchange area, the heat exchange area is constructed as a part of the outdoor air duct, and the first-stage heat exchanger core, the first side air duct, and the second-stage heat exchanger core are sequentially arranged along a flow direction of the outdoor air duct; an inner cavity of the first-stage heat exchanger core and an inner cavity of the second-stage heat exchanger core each are constructed as a part of the indoor air duct; and the first side air duct is configured to permit air exchange between the heat exchange area and a region outside of the composite cooling system, wherein the first side air duct is configured to permit air in the outdoor air duct to flow from the first-stage heat exchanger core to the first side air duct, mix with air in the first side air duct, and flow from the first side air duct to the second-stage heat exchanger core; wherein the indoor air duct comprises an air inlet section and an air return section, in a flow direction of the indoor air duct, the air inlet section is located at a front end of the heat exchange area, the air return section is located at a rear end of the heat exchange area, the first side air duct further communicates with the air inlet section and the air return section, a first air valve is disposed between the first side air duct and the air inlet section, a second air valve is disposed between the first side air duct and the air return section, and the first side air duct is also constructed as a part of the indoor air duct. a first-stage heat exchanger core, a second-stage heat exchanger core, and a first side air duct are disposed in the heat exchange area, the heat exchange area is constructed as a part of the outdoor air duct, and the first-stage heat exchanger core, the first side air duct, and the second-stage heat exchanger core are sequentially arranged along a flow direction of the outdoor air duct; an inner cavity of the first-stage heat exchanger core and an inner cavity of the second-stage heat exchanger core each are constructed as a part of the indoor air duct; and the first side air duct is configured to permit air exchange between the heat exchange area and a region outside of the composite cooling system, wherein the first side air duct is configured to permit air in the outdoor air duct to flow from the first-stage heat exchanger core to the first side air duct, mix with air in the first side air duct, and flow from the first side air duct to the second-stage heat exchanger core; wherein the indoor air duct comprises an air inlet section and an air return section, in a flow direction of the indoor air duct, the air inlet section is located at a front end of the heat exchange area, the air return section is located at a rear end of the heat exchange area, the first side air duct further communicates with the air inlet section and the air return section, a first air valve is disposed between the first side air duct and the air inlet section, a second air valve is disposed between the first side air duct and the air return section, and the first side air duct is also constructed as a part of the indoor air duct.