Abstract: Based on the binocular vision theory, the effects of the convergence angle and the focusing difference of the binocular optical system with large exit pupil were analyzed.Furthermore,the factors and the misunderstanding of design which could influence the visual comfort were also analyzed. It is pointed out that different pupil distances and the condition of nonzero visibility observation instrument should be considered. The result shows that the convergence angle should be controlled between 0~4 m-1 ,and the binocular focusing difference should be 0~2 m-1 which is the smaller,the better.