证明了对于在真空中稳态传输的光束来说，其光束截面角动量的轴向分量、角动量平方的轴向分量都是与z无关的守恒量；此外还把这一结果推广到傍轴旋对称ABCD光学系统；并对一些与角动量有关的问题进行了讨论。

In this peper, we prove that, for any stable propagnting light beam in vacuum or in the symmetrically parexial ADCD system, the z components of beam cross-section angular momentum and angular momentum square are both.invariables which are independent of the position of the cross-section. Furthermore, we still discuss some characters of light beam which are relative to angular momentum.