An outdoor system for an air conditioner may include at least one outdoor unit, the at least one outdoor unit including a compressor; an outdoor heat exchanger; a pair of two-stage compression lines that extends to outside of the outdoor system; a pair of connection lines that extends to the outside of the outdoor system and communicates with an indoor unit; and multiple valves that open/close the pair of two-stage compression lines and the pair of connection lines when the outdoor system is operated in a one-stage heating mode or a two-stage heating mode.