A cooling fan variable-frequency control system for a power transformer includes a control device and a fan unit. The control device includes a microprocessor that is set with a first temperature range, a second temperature range higher than the first temperature range, and a fan switching time. The microprocessor is connected to a temperature sensor for detecting a temperature of an insulating oil in a power transformer. A variable-frequency controller and a fixed-frequency controller are connected to the microprocessor and are connected to a switching controller. The fan unit is connected to the switching controller and includes first and second fans. The microprocessor controls the switching controller according to the fan switching time to thereby control the variable-frequency controller to connect with the first fan or the second fan. The first fan and the second fan can operate at a fixed frequency and at a variable frequency.