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Tin-based perovskites are one of the most promising candidates for the development of lead-free perovskite solar cells (PSCs) and have attracted lots of attention. Despite the extensive research efforts, the performance of tin-based PSCs (Sn-PSCs) still lags far behind the lead containing counterparts due to the poor stability and self-p-doping. Here, we perform first-principles density functional theory calculations combined with non-adiabatic molecular dynamics simulations to unveil the origins of the instability of tin iodine perovskite when exposed to O2. It is found that O2is more thermally favorable to be adsorbed on iodine-vacancy (VI) defect sites in the defective surface rather than the pristine surface, and the generation of peroxide species on the VIsites dramatically accelerates the structural decomposition. The presence of VIdefects on the surface of CsSnI3decreases the band gap by inducing a local shallow state around conduction band minimum, significantly accelerating the electron–hole recombination. The adsorption of O2on VIsite slightly increases the band gap compared to that of the pristine one and decreases the influence of VIon the surface’s optoelectronic properties and the electron–hole recombination rate but significantly accelerates structural decomposition by weakening the defect tolerance ability of the [SnI6]4–octahedra as well as the carriers’ relaxation. The increased structural instability in combined VIand O2points to the surface VIdefect passivation as the main optimization scheme for efficient and stable Sn-PSCs.