Abstract: This paper presents a method to control the gain and measure the amplitude exactly for underwater sound pulse with a wide dynamic range, in which a loop composed of gain controlled unit and ndcrocomputer is employed. This method also has a satisfactory result when there are distortion with signal envelop and fluctuation with signal amplitude. The basic mathematical model of instantaneous amplitude-gain-control and amplitude measuring, the hardware strueture are presented. The calibration method of the system for re-transmitting (response) as required sound level is given as well.