Yao is a mathematical expression, which refers to the conservation of probability in the process of microscopic particle scattering and reaction. Matter in the micro-world has wave-particle duality, and the total probability of finding particles in the whole space at time t is equal to 1. If microscopic particles cannot be produced and annihilated, then the wave function satisfies the normalization condition at a certain moment, and at any moment, the wave function will remain normalized (probability conservation). It describes the principle of material immortality in microscopic process.