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Nonradiative charge recombination comprises a main pathway for energy losses that impedes the performance of all-inorganic perovskite solar cells. Grain boundaries (GBs) defects are unavoidable in low-temperature solution-processed perovskite polycrystalline films, but their role remains unclear. By performing ab initio nonadiabatic (NA) molecular dynamics simulations, we illustrate that electron–hole recombination in CsPbBr3 takes place over 100 ps, achieving a good agreement with experiment. Introduction of GBs into CsPbBr3 accelerates the recombination, while GBs doping with chlorine notably slow it down. Importantly, GBs do not create deep electron traps because they only narrow the band gap slightly. GBs localize electron wave functions at boundaries and activate additional phonon modes, leading to an enhanced NA coupling and a shortened coherence time. Consequently, the interplay between the three competitive factors accelerates the recombination by a factor of 2. Chlorine doping diminishes the mixing of electron and hole wave functions and reduces the NA coupling, which also shortens the coherence time further by introducing higher-frequency phonons, notably delaying the recombination. Our study establishes the atomistic mechanism that the acceleration and retardation in electron–hole recombination induced by GBs and chlorine doping in CsPbBr3 perovskite, providing new insights to improve the material properties via passivating the GB by chemical doping.