In order to achieve the 100 meters far away object measurement of large radio astronomical telescope, visual measurement system has been established. The arithmetic of object detection and stability of the measurement system has been researched. First, the CCD camera obtains the image information of the reflected target 100 meters far away. The target image is interpolated by using spline function and the edge information of the target is detected with Canny algorithm. Then, the position of target is measured by adopting ellipse fitting algorithm. Consequently, the image plane coordinates of the target is acquired. The validity of the detection algorithm is appraised by adopting a plane calibration jig. Finally, in the outdoor conditions, the stability of the target measurement is researched at day and night. Experiment results show that the target detection algorithm has the measurement precision of sub-pixel and the stability precision of 100 meters distance target measurement is 0.08 pixels. It can atisfy the 100 meters distance coordinates measurement of large radio astronomical telescope and has better anti-jamming capability.