The target plane of the space tridimensional mapping camera will be deviated from the focal plane, under the condition of space environment and vibration, impact when satellite is launching, and image resolution ratio will be descended because of defocusing. For tridimensional mapping camera, principal point position and focal length variation of the camera affect positioning accuracy of ground target. In order to revise the shortage of photoelectric encoder utilization, the measuring and focusing method was put forward based on astigmation. A quadrant detector was adopted to measure the astigmation caused by the deviation of the target plane, refer to calibrated relation between the target plane poison and the asrigmation, so the deviation vector of the target plane can be obtained. This method includes all factors caused deviation of the target plane, and experimental results show that the focusing resolution of mapping camera focusing mechanism based on astigmatic method can reach 0.025 mm.