There is provided a system and method for controlling a temperature of a refrigerant in an air conditioner, in which a supercooling degree and/or a superheating degree can be secured by controlling a difference in refrigerant temperatures of a pipe connecting one or more indoor units to one or more outdoor units, and a flow of a specific refrigerant. The system includes: one or more indoor units; one or more outdoor units; a high-pressure pipe and a low-pressure pipe for connecting the indoor units and the outdoor units; and a refrigerant temperature control unit coupled to the high-pressure pipe and the low-pressure pipe, for performing a heat exchange with respect to flowing refrigerants by coupling an inner pipe to an outer pipe, the inner pipe passing through the another pipe. The refrigerant temperature control unit is installed in one side of the high-pressure or low-pressure pipe and senses a supercooling degree and/or a superheating degree and increasing/decreasing a refrigerant inlet flow to the outer pipe through a bypass passage, which couples the outer pipe to a specific pipe, so as to make the sensed supercooling or superheating degree equal to a target value.