Theoretical analysis and numerical simulation calculations have been carried out to explain the dependence of ruby laser output on temperature, solvent and impurity of cryptocyanine solution used as the passive-Q-switch. The results show that the absorption of excited state is the main cause of the temperature, solvent and impurity effects, especially the transition of S1-S2 Optical bleaching of cryptocyanine depends mainly on the saturation of S1-S2 transition, and is influenced by the population of the first triplet state as well. Therefore the thermal stability of the cryptocyanine-switched ruby laser is largely controlled by the solvent quenching rate of singlet states.