An electric tool has a motor, an implementing member driven by the motor for driving a working element and a controlling circuit for controlling the electric tool. The controlling circuit includes a current detecting module for detecting the current flowing through the motor; a microprocessor for memorizing and storing a current value; an user interface module for an user to select working modes, and the working modes at least comprising a learning mode for recording the current value after the implementing member has driven the working element and a constant torque mode for driving other working elements having the same size based on the recorded current value; and a driving module for driving or stopping the torque output of the motor. The electric tool can achieve the function of electronic clutch so as to drive screws having the same size with substantially the same effect. The present disclosure also discloses a controlling method by using the above controlling circuit.