Considering the problem that the parallax of an optical sight with small field-of-view can't be tested with traditional method using collimator tube, we established a parallax testing system by using specialized clamp, two-axis displacement platform, a photoelectric theodolite and optical platform. The photoelectric theodolite is taken as the measurement basis. The maximum parallax is tested when the theodolite is on the extreme position of exit pupil in azimuth/pitching direction. Experimental result showed that the system is stable, and the method is easy in operation and has high measurement accuracy.