A method for controlling an auto-balancing system of an optic disk in an optic disk drive is proposed. As the rotary speed of a spindle motor is not faster than a critical speed, a dither signal of high frequency and micro amplitude is added to a command signal for speeding a maximum rotary speed for reducing friction force, and then the balancing mass successfully moves to a position with a reverse phase opposite to an unbalance vector of the optic disk. After the rotary speed of the spindle motor is higher than a critical speed, the dither signal and then speeding the spindle motor is removed and the motor is accelerated to a maximum rotary speed.