Large aspheric mirrors with diameter over 200 mm are widely used in space communication system, large-scale astronomical telescope and X-ray facilities. However the measurement of these mirrors with large slope is a long-term problem. Scanning deflectometry method using autocollimator with rotary stage was proposed for measuring mirrors with large slope. The repeatability of the method has been confirmed as tens of nanometers. However, the absolute accuracy of the method is not confirmed since the distance between the rotation center of the rotary stage and the mirror under measurement is difficult to measure, which strongly affects the measurement accuracy of scanning deflectometry method. In this paper, we have proposed a triangulation method with CMOS sensor for measuring the distance from the rotation center to the measurement table (marble surface). The error analysis shows that the measurement uncertainty of this measurement method is about 40 microns. The repeated experiments show that the measurement accuracy of the proposed method is 49.7 microns, which meets the requirements of large mirror measurement with tens of nanometers’ uncertainty. This method also provides a new solution for the position measurement of the rotation center of the large scale rotary device.