A control method for a cooling system is provided. The system includes a vehicle operation state detecting portion having an ambient temperature sensor, first and second coolant temperature sensors, a coolant control valve unit that adjusts opening rates of first, second, and third coolant passages and a controller. The method includes determining whether an output signal of the ambient temperature sensor satisfies a predetermined an ambient low temperature driving condition and whether an output signal of the first coolant temperature sensor satisfies a predetermined first low temperature driving condition when the output signal of the ambient temperature sensor satisfies the predetermined the ambient low temperature driving condition. The coolant control valve unit opens the first and coolant passages and closes the second coolant passage when the output signal of the first coolant temperature sensor satisfies the predetermined first low temperature driving condition.