High power pulse laser and large aperture telescope are usually used for getting signals from higher altitude in lidar observation, which brings the detector saturation caused by strong backscattering from low altitude. The mechanical chopper improves the lidar performance by cutting off the low altitude noise in optical method and passing the backscattering signals from higher. The working principle of the mechanical chopper is analyzed, and the chopper unit is built up. The testing data are agreed well with the theoretical results. The application of the chopper in lidar detection system can solve the problems of detector saturation and blinding caused by low altitude backscattering, so as to increase the detection range and the measuring accuracy.