The angular variable mirror (AVM) to be used to control the power balance of beams of “SHENGUANG Ⅱ” system will affect the light beams, including vertical displacement of optical axis, alteration of equivalent length of optical path, additional optical distance, and the curve of wave surface. By the optical matrix, the effects have been analyzed quantitativelly, and point out that equivalent length of optical path is the different concept from optical distance. The application of AVM in practice is discussed.