Provided is method for automatic audio gain modulation and a related apparatus. In order to prevent the unstable signals as continuously increasing or decreasing the gain when the signals hover around an upper threshold, it's featured that the method is to predict the signal intensity as pre-adding a predict gain in a hold time before outputting signals. By determining whether the prediction exceeds a predetermined threshold or not, the suitable and adjustable gains can be obtained without exceeding the predetermined threshold. In the automatic gain control mechanism, it's to decrease the gain gradually as automatically entering an attack time, or to increase the gain gradually as performing a conventional auto-gain control procedure including entering a release time. The claimed method can modulate the gain automatically by predicting the gain, and confine the signals under a threshold. In addition to avoid sawtooth phenomena, the method can stabilize the signals.