A fan controlling device for a frequency converter, which is applied to a high-power frequency transforming system that installs fans according to the heat dissipating requirements and their switching operation, failure detection and control, and the fan controlling device includes a frequency transformer and a plurality of direct current fans connected to the frequency transforming system, and each fan is installed at a desired position for dissipating heat in a casing of the frequency transformer by locking, sliding tracks, drawing or latching and each fan is connected to a fan controlling device, and a sensor of the fan controlling device detects the temperature of each set temperature point for switching the fans, controlling the operating number and speed of the fans, so as to improve the quantity, position, operability of installed fans as well as reducing the power consumption when the fans are at an idle state.