In the LiDAR system, due to the variation of the target distance and reflection characteristics, the peak value of the echo signal fluctuates in a relatively large range, which leads to a big fluctuation of the arrival time of the echo determined by the constant ratio timing discriminator circuit, and the resulting walking error affects the ranging accuracy and measurement range of the system. In this paper, a constant fraction discriminator with adaptive gain control is designed, thus the gain of the amplifier circuit can be adjusted quickly and automatically based on the level of the echo signal, which can adapt to the target measurement in a large dynamic range. The results illustrate that the designed circuit adopting automatic gain and constant fraction discriminator component is able to automatically configure and amplify the circuit gain within 25ns. When increasing duration of input signal to 3ns and the voltage dynamic range is stabilized around 29dB, the automatic control system adopted automatic gain design can effectively reduce the fluctuation range of dynamic error and the discrimination error of echo signal to 9dB and 0.42ns respectively, reducing the walking error significantly.