The stereo imaging of a long focus off-axis three-mirror optical system on orbit was simulated in a laboratory, and the imaging possibility of the system on orbit was verified by image processing and image analysis. A single point imaging method was proposed for the outdoor stereoscopic imaging of the long focus off-axis three-mirror mapping camera. In this method, the single cameras was used to image the target through rotating an experiment table. The target was rotated according to a planned camera rendezvous angle and the angle was measured exactly. Then, the intersection angle of the two line array cameras was simulated by rotating the target. In the outdoor stereoscopic imaging, the target was designed based on the orbital altitude, object distance, elevation accuracy and planned precision. Finally, the image methods were verified and the measuring accuracy for exterior orientation elements and the angle between two cameras were analyzed. Obtained results demonstrate that the single point imaging method has practical values and can be used in the feasibility verification of stereoscopic imaging of long focu off-axis three-mirror cameras.