Since the cableless acoustic logging instrument cannot transmit the wave train data to the surface for processing in real time, the automatic gain control of the acoustic instrument cannot be carried out by the surface system. In order to obtain the appropriate amplitude of the first wave and prevent the first wave signal from being too strong or too weak, which will affect the extraction of the time difference data, the sonic instrument needs to realize the automatic gain control downhole. The automatic gain control technology includes the extraction of the first wave and real-time gain adjustment. This design adopts the STM32L496 microcontroller for the extraction of the first wave, and the real-time gain control is carried out by two-stage AD602. Through the short window–long window energy ratio of the first wave detection dynamic control gain size to achieve the requirements of downhole automatic gain control, the instrument can quickly sense the changes in the acoustic signal, and quickly adjust the gain value to ensure the continuity and stability of the signal. Downhole environment is complex and changeable, the stability of the instrument puts forward high requirements. The downhole automatic gain control technology can adapt to various downhole environments and ensure that the instrument remains stable during the long working time. The appropriate amplitude of the first wave of the acoustic wave is obtained without manual intervention, which makes the acoustic logging instrument able to correctly obtain the time difference of the formation acoustic wave.