A variable frequency controlling system of a shredder is provided. The variable frequency controlling system includes a thickness sensor, a speed-variable motor and a controlling module. The controlling module is electrically connected to the speed-variable motor and the thickness sensor, and includes a look-up table. The thickness sensor is used for detecting a thickness of an article fed into the shredder. The speed-variable motor is operated at a variable rotating speed. When a thickness-detecting signal is received by the controlling module, the operating data corresponding to the thickness-detecting signal are searched from the look-up table and outputted from the controlling module to the speed-variable motor, thereby controlling the speed-variable motor to be operated at a proper rotating speed. As a consequence, a power-saving purpose is achieved by the variable frequency controlling system.