La1-xSrxFeO3-δ is one of the common cathode material for SOFC. The ratio of the mixed multivalent cations, temperature, oxygen partial pressure, and phase stability jointly determine the oxygen nonstoichiometry, δ, which controls the functionality of LSF. In this study, a method combining density functional theory(DFT) with thermodynamics was developed to predict δ in LSF at SOFC operating conditions and especially when δ is too high for dilute approximation. The predicted δ agrees well with experimental data in a broad temperature range. The phase stability of cubic, orthorhombic, rhombohedral, tetragonal, and Brownmillerite were calculated for LSF with different x and δ. Fe can form four, six, and five coordinated Fe-O complexes in these structures. It was found that the oxygen vacancies are generated from Fe-O complex with higher charge state. This suggests that stabilizing Fe4+ in Fe-O octahedral via doping can increase δ and the performance of LSF.