Abstract: An assembling method for off-axis parabolic mirror of a high-precision polarimeter based on ellipse detecting was put forward. Using the reversing light path, the light field diaphragm could be imaged to be a focusing spot on camera. The ellipse detecting means based on least square method was adopted to trace the focusing spot and get the moving track of the center of the spot,so as to measure the shift-error of the optic axis. A special assistant-software was developed to control camera and process image. The spots moving track was shown on the software interface, thus technicians can judge how to adjust mirrors position according to how the track is changing. It is verified that this method can guide technician assembling effectively and the shift-error of the optic axis can be detected out accurately. Finally, the assembling error is corrected from 1.33 to 0.3 which meets requirement.