An outdoor system of an air conditioner comprising two outdoor units (100, 200), wherein the two outdoor units (100, 200) include: a first outdoor unit including a first compressor (120, 130), a first outdoor heat exchanger, a first main four-way valve disposed at an inlet side of the first compressor (120, 130), a first auxiliary four-way valve disposed at an outlet side of the first compressor (120, 130), a first heat exchanger input/output line, and a first outdoor system connection line; a second outdoor unit including a second compressor (220, 230), a second outdoor heat exchanger, a second main four-way valve disposed at an inlet side of the compressor (220, 230), a second auxiliary four-way valve disposed at an outlet side of the second compressor (220, 230), a second heat exchanger input/output line, and a second outdoor system connection line; a pair of two-stage compression lines (122, 222) extending to the outside of the outdoor units (100, 200); wherein the valves (150, 160, 250, 260) are configured to open and close the heat exchanger input/output lines (102, 202), the outdoor system connection lines (124, 224), and the pair of two-stage compression lines (122, 222); and wherein, when the outdoor unit (100, 200) is switched from any one of the one-stage heating mode and the two-stage heating mode to the other thereof, any one of the main four-way valve (150, 250) and the auxiliary four-way valve (160, 260) is switched, wherein in the one-stage heating mode, refrigerant flowing to the first heat exchanger input/output line and the second heat exchanger input/output line are compressed by the first compressor (120, 130) and the second compressor (220, 230), flowing to the indoor unit along the first outdoor system connection line and the second outdoor system connection line, and in the two-stage heating mode, refrigerant flowing to the first heat exchanger input/output line and the second heat exchanger input/output line are sequentially compressed in the first compressor (120, 130) and the second compressor (220, 230), thereby flowing to the indoor unit along the second outdoor system connection line. a first outdoor unit including a first compressor (120, 130), a first outdoor heat exchanger, a first main four-way valve disposed at an inlet side of the first compressor (120, 130), a first auxiliary four-way valve disposed at an outlet side of the first compressor (120, 130), a first heat exchanger input/output line, and a first outdoor system connection line; a second outdoor unit including a second compressor (220, 230), a second outdoor heat exchanger, a second main four-way valve disposed at an inlet side of the compressor (220, 230), a second auxiliary four-way valve disposed at an outlet side of the second compressor (220, 230), a second heat exchanger input/output line, and a second outdoor system connection line; a pair of two-stage compression lines (122, 222) extending to the outside of the outdoor units (100, 200); wherein the valves (150, 160, 250, 260) are configured to open and close the heat exchanger input/output lines (102, 202), the outdoor system connection lines (124, 224), and the pair of two-stage compression lines (122, 222); and wherein, when the outdoor unit (100, 200) is switched from any one of the one-stage heating mode and the two-stage heating mode to the other thereof, any one of the main four-way valve (150, 250) and the auxiliary four-way valve (160, 260) is switched, wherein in the one-stage heating mode, refrigerant flowing to the first heat exchanger input/output line and the second heat exchanger input/output line are compressed by the first compressor (120, 130) and the second compressor (220, 230), flowing to the indoor unit along the first outdoor system connection line and the second outdoor system connection line, and in the two-stage heating mode, refrigerant flowing to the first heat exchanger input/output line and the second heat exchanger input/output line are sequentially compressed in the first compressor (120, 130) and the second compressor (220, 230), thereby flowing to the indoor unit along the second outdoor system connection line.