As the core functional layer of perovskite solar cells, the serious issues of the CH₃NH₃PbI₃ film related to the long-term stability and its rapid degradation when exposed to the environment should be investigated deeply. In this study, the variation of phase construction, light absorption ability and fluorescence quenching ability during the long time aging process have been monitored. The results show that the degradation process is composed of the original serious fluorescence quenching and the lag behind phase decomposition. Then, the intrinsic physical mechanism has been obtained by the first-principles simulation of defect properties, which shows that the original serious fluorescence emission quenching is attributed to the deep level defects with low formation energies (such as V_Pb and I_Pb); meanwhile, the lag behind phase decomposition is caused by the easy ionic diffusion; for example, the diffusion activation energy of the iodine ion is 0.286 eV. The results illustrate that both the defect passivation and prevention of the ion diffusion are necessary for achieving a stable perovskite film.