A double-helical gear is a kind of special gear which can be used on large power transmission devices. It can work perfectly in the overload and high speed transmission system. But the special structure requirements of teeth make the machining process difficult. Especially there is a strict requirement on the symmetry between the teeth and the central shaft, and it is difficult to be tested. Based on the analysis of symmetry for double-helical gears, the main measurement factor was found, and a new test method on CMM was put forward in this article.