Front view distance image is achieved through the scanning of laser radar. From the distance image, the information of obstacles can be achieved, which is a basis of terrain follow and terrain avoiding (TF/TA) of the aircraft on the inertia navigation system (INS). According to the theory of laser radar imaging, the front view imaging formula of laser radar is deduced, and the algorithm which identifies barrier from the 3D distance image is presented. In the end, the image of 3D distance is achieved by the simulation on the 3D terrain map, and the experiment of obstacles detection is completed with the 3D distance image.