For a direct-detection 3D imaging laser radar,the use of photon counting detector could greatly enhance the detection efficiency of the receiver.This paper studies the 3D imaging lidar using Geiger Mode APD.In this paper,the detection model for this laser radar is analyzed.Based on this analysis,the detection probability and rangefinding precision are discussed.Then a photon counting 3D imaging brass board system is described,and some experimental results obtained by this system are presented and analyzed.