从施特恩-盖拉赫实验测量的结果电子自旋只有两种取值出发，在不引入自旋算符的具体矩阵表示的情况下，利用矩阵求迹，导出自旋算符和轨道角动量算符满足完全相同的对易关系，从而说明自旋与轨道角动量同类，属于角动量. 区别于历史上和国内外量子力学教材中直接将自旋归为角动量，本文尝试用“物以类聚”的逻辑对自旋是角动量进行讨论.

Only taking into account the results measured in Stern -Gerlach experiment，we obtain that the commutator relations of spin operators are similar to the ones of orbital angular momentum operators by using the rule of taking trace of matrix without introducing the matrix representation of spin operator. Then we conclude that the spin and orbital angular momentum belong to the same kind: angular momentum，which is different from the original concept of spin introduced in history and the one in quantum mechanics textbooks. We attempt to discuss about that the spin belongs to angular momentum with the philosophy“birds of a feather flock together”.