Disclosed is an apparatus and method for preventing liquid refrigerant accumulation in an accumulator of an outdoor unit of an air conditioner. The method includes the steps of: determining a state of a compressor; if the compressor is in standby mode or off state, turning on a four-way valve; determining whether the state of the compressor in standby mode or off state continues longer than a predetermined amount of time; and if the state of the compressor continues longer than the predetermined amount of time, closing an expansion valve. Particularly, high-pressure refrigerant flows in the closed expansion valve while low-pressure refrigerant flows in a check valve through the four-way valve. The apparatus includes a suction pipe connected to spray oil of the accumulator to the compressor; a liquid recovery pipe with one end being connected to the suction pipe and the other end being connected to the accumulator; and a liquid recovery valve for controlling opening/closing of the liquid recovery pipe, in which a compressor of the air conditioner in cooling or heating mode is operated and a liquid recovery valve for controlling a liquid recovery pipe connected to the compressor and an accumulator is opened to reduce oil in the accumulator. According to the present invention, when the compressor is off, it is possible to prevent liquid refrigerant accumulation by changing refrigerant flow line to the accumulator and the refrigerant at the lower end of the accumulator can be recovered.