As a frequent part of optical systems, the lens group plays an important role in the large aperture optical system, but the lens group can not be tested directly after alignment. The disadvantage during common measurement of the lens group was analyzed. With adding one simple optical element, a new method for testing the whole lens group after alignment, which solves the test problem of lens group applied in the large aperture optic systems. Two practical optical systems were given, and the design and accuracy demand of the lens group were analyzed. Next, the lens group of the practical optical systems with the new method were tested, and the comparison of analyzed results and test results proved the reliability and precision of this method, which derives that the precision of measurement is better than 0.01λ. At last, the optical quality of lens group after final alignment meets the design requirement.