This article analyze the physics of up-conversion process excited by CW laser. The key metastable state is very important because it initiates the up-conversion process.By analyzing the dynamic process of the key metastable state, a qualitative “population branching ratio β” model is proposed. The physical significance of this method is that it helps to understand the general characteristics of up-conversion.Further analysis indicates, spontaneous emission Ａ and non-radiative relaxation Wn do act simultaneously.And the reason why the up-conversion fluorescence of quartz,pentaphosphate non-crystalline is about ten thousand times smaller than that of fluoride glass is that their multi-phonon nonradiative relaxation rate Ｗn is much larger than the spontaneous emission rate Ａ. We also report the benefit value and the condition Wn＜A of phonon for up-conversion.