This paper presents that pumping power change of Nd: YAG laser will induce not only gain change but also loss change. As a result, it induces oscillating mode change, laser cavity unstable, and the complicated change of laser output power. On the basis of a large theoretical calculation and experimental support, the paper shows that the plane-parallel cavity is more suitable for the multimode operation and the plane-convex or concave-convex cavity for the single mode operation