Abstract: The temperature of cooled water could not be maintained in a desired range by a fan running at constant-speed in a cooling tower under different loads and environmental temperatures. The system was modified with frequency conversion technology, a thermistor was used to detect the temperature of cooled water, and control the output frequency of a frequency converter via the temperature signal, thereby to regulate the rotating speed of the fan and the heat dissipation of the cooling tower, and control the temperature of cooled water within the tolerable range.