Oxygen vacancy (VO) formation in perovskites plays an important role in improving their functional applications. Using density functional theory calculations, we investigated the effect of sulfur (S) doping on VO formation in LaBO3 (B = Fe, Co, and Ni) perovskites, considering the HS, IS, and LS states of Co ions in LaCoO3 to examine the influence of spin states. Our results show that the weaker electronegativity of S2- than that of O2- leads to a decrease in the magnetic moment of B atoms directly adjacent to the substituted S and an increase in the electrical conductivity of insulating systems. Formation energy (Ef) calculations suggest that S doping is beneficial for VO formation. In particular, VOs are more likely to form at oxygen positions adjacent to S ions. Moreover, upon S doping, spin state transition is not a necessary condition to lower the Ef. Instead, the main reason for reducing the Ef of VOs is the decreased relaxation energy of the lattice following VO formation. Therefore, we revealed a common mechanism for S-doping-promoted VO formation, which could be extended to other perovskites.