An automatic gain control method includes receiving a sequence of multiple digital data, and calculating a plurality of signal values corresponding to the respective voltage values of the digital data, such as multiple peak-to-peak voltage values or power values, so as to optimize a gain according to variations in the output values. The gain optimization includes updating a reference value according to the signal values. If the reference value is less than a minimum threshold, the gain is increased to cause the reference value to reach the minimum threshold. The gain optimization also includes analyzing a clipping rate according to the signal values. If the clipping rate is equal to zero, then the gain is adjusted up. If the clipping rate is greater than zero, then the gain is adjusted down, such that the clipping rate is decreased to approach to zero.