The focal length of lens group can be measured by nodal slide. The advantage of using nodal slide is intuitive. Without nodal slide, we can also measure by Newton formula. When using Newton formula, we need to know the position of focus, object point and image point. The usual method is to judge the clarity of the image. Different people's judgment is different，and the error is large. In this paper, the method of object-image parallax comparison is used to judge. First, the whole measuring system is introduced, then the pinpoint system is introduced. Finally, the focus, object point and image point positions are measured by the methodof object-image parallax comparison. Finally, the focal length of lens group is obtained. The relative deviation is 0.13%. At the same time, the focal length of lens group are measured by a nodal slide， and relative deviation is 0.45%. It can be seen that the results measured by the method of object-image parallax comparison are more accurate.