The method for expanding an air bag includes the steps of: detecting a variation of a deceleration of a vehicle during a vehicle collision and applying the detected deceleration signal to a microcomputer. Analyzing a collision waveform generated according to the deceleration signal and determining an impact degree of the vehicle. Temporarily stopping an operation of the deceleration detector at the microcomputer. Initializing an initial collision waveform while the operation of the deceleration detector is stopped, the initial collision waveform being inputted before the microcomputer stops the operation of the deceleration detector. Re-operating the deceleration detector at the microcomputer, detecting again a variation of the deceleration of the vehicle at the deceleration detector, and applying the newly detected deceleration signal to the microcomputer. When the collision waveform, generated according to the applied deceleration signal, reaches a set value, activate an air bag driving unit at the microcomputer.