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To understand the instability of Sn-based perovskite, CH₃NH₃SnI₃, photoelectron spectroscopy with synchrotron radiation and theoretical calculations, such as density functional theory and ab initio molecular dynamics, were performed. Findings from this experimental and theoretical study highlight the crucial changes of surface-chemical states, the broken chemical bondings in Sn–I, and the depletion of a CH₃–NH₃⁺ cation on the surface region. The material instability origin of Sn-based perovskite can be explained by the chemical state instability in the surface.