Photosynthetic water oxidation and oxygen evolution are fundamental biological processes in nature, with significant and challenging scientific issues centering on the oxygen-evolving complex, as well as some conundrums and controversies in this field. Theoretical and computational approaches are very important to elucidate the inherent mechanisms, which should also be conducive for the development of the artificial functional materials applied to water splitting. Based on the cutting-edge progress and using density functional theory as the tool, systematic investigations were carried out from the aspects of interaction between ammonia and the Mn cluster, and the mechanism of O-O bond formation. The related achievements make sense to the mechanism of photosynthetic oxygen evolution.