镜面在入射面内旋转时，其反射光线的角度变化是镜面转动角度的两倍；当镜面存在面外旋转时，其反射光线的角度变化难以定量描述. 本文结合光线反射定律与解析几何相关知识，研究了当镜面在三维空间内自由旋转时，其反射光线的角度变化规律，并得出了与入射角、入射面内旋转角、面外旋转角相关的一般表达式. 结果表明，当不存在入射面内旋转时，反射光线的角度变化随面外旋转角度变化的变化速度是最快的；当入射面内旋转角度增大时，反射光线的角度变化随面外旋转角度变化的变化速度逐渐减小，直至为0. 这一研究结果，将有助于理解自由空间中，镜面旋转时反射光线的变化规律，为光线在快速反射镜、扫描系统、数字微镜器件中的传播提供理论支撑.

When the mirror rotates within the incident plane, the angle change of the reflected light is twice the mirror's rotation angle. However, when the mirror rotates out of the plane, the angle change of the reflected light is difficult to describe quantitatively. This paper combines the law of light reflection and the geometry to study the angle change of the reflected light when the mirror rotates freely in three-dimensional space. A general expression is obtained, which is related to the incident angle, the in-plane rotation angle, and the out-of-plane rotation angle. The results show that when there is no rotation within the incident plane, the angle change of the reflected light varies most rapidly with changes in the out-of-plane rotation angle. As the in-plane rotation angle increases, the angular change of the reflected light with the out-of-plane rotation angle gradually decreases until it reaches zero. These findings enhance the understanding of the behavior of reflected light when the mirror rotates in free space and provide theoretical support for light propagation in fast reflectors, scanning systems, and digital micromirror devices.