In order to avoid the shortcoming of the passive gain control method in the 3D imaging lidar (light detection and ranging), we propose a new gain control method, which can adjust the gain of the amplifying circuit according to the target distance. This method complies with the principle that the laser echo amplitude is inversely proportional to the square of the target distance. In addition, to simplify the complexity of the gain control module, we propose a simple implementation method based on the charging process of a capacitor. Firstly, the theoretical waveform of the proposed gain control method and the gain control error are analyzed and simulated. The results indicates that when the gain ranged from 1 to 100, the maximum of gain error is less than 28% in the whole target distance range, and the gain error is less than 5% in the most target distance range. Based on this method, a new gain control and amplifying circuit has been developed, which is mainly composed of an amplifying module and a gain control module. The gain control module is used to generate a gain control voltage and apply the voltage to the gain control of the amplifying module. Finally, some experiments have been carried out to verify the entire circuit functions and performances. The experimental results show that the output signal amplitude keep constant on the whole when the target distance is changing. The pass band of the circuit ranges from 0.33 MHz to 150 MHz, and the maximum gain is 316.