The present invention relates to an active brake circuit for a fan with backup power, which comprises a protection circuit, a motor driving unit, a motor, a fan controller, and at least one bypass unit. The protection circuit receives an input power source. The motor driving unit connected between the protection circuit and the motor outputs a driving signal to the motor. The fan controller connected to the motor driving unit and the protection circuit outputs a control signal to the motor driving unit. The bypass unit is between the input power source and the motor driving unit such that the bypass unit passes the input power source to the motor driving unit. When the protection circuit disconnects the input power source to the motor, the motor driving unit outputs the driving signal to the motor such that a short circuit is formed to brake the motor.