According to the research of traditional parallax measuring techniques, a parameter test method based on the auto-focus technology was proposed, and the principle of auto-focus and its composition block diagram were introduced. Then, in consideration of the specific requirements of small arms for sight performance, a parallax test system was designed combined with the auto-focus technology and the auto-focus processes before or after the measuring sight placed in the test system were described in detail. Furthermore, the parallax parameters were obtained according to the difference of diopter. Five experiments were performed on the requirements of the actual optical system, and the obtained parallax parameters were analyzed. Results show that the measurement accuracy of parallax parameters is 0.029 6 m-1, which achieves the technical specification less than 0.1 m-1 and guarantees the measuring accuracy.