An automatic gain controller and the controlling method thereof are disclosed. The automatic gain controller includes a first multiplexer for receiving an input signal and a gain and generating a first output, a second multiplexer for receiving a signal time constant and a gain time constant and generating a second output, a filter electrically connected to the first multiplexer and the second multiplexer for generating one of a signal-energy and an actual gain in response to the first output and the second output, a signal-energy processing device electrically connected to the filter, the first multiplexer and the second multiplexer for generating the gain and the gain time constant in response to the signal-energy, and a multiplier electrically connected to the filter for multiplying the actual gain by the input signal to generate an output signal.