A method for controlling a multi-unit air conditioner is disclosed which is capable of preventing frost from being formed on the outer surface of a refrigerant line. In order to control an air conditioner including an outdoor unit, a plurality of indoor units corresponding to the out door unit, and a plurality of refrigerant lines for connecting the indoor units to the outdoor unit, respectively, the method includes the steps of measuring respective states of the refrigerant lines using a measurement sensor, determining respective states of the refrigerant lines, based on measurement data obtained by the measurement sensor, and controlling an operation state of an operating one or operating ones of the indoor units, in accordance with the determined state of the refrigerant line associated with the operating indoor unit or each operating indoor unit.