In this paper, we present the ab initio calculations of S substituting for P point defects (Sp) in KDP crystal. The electronic structure and the relaxing configuration of Sp are studied. The properties of density of states and band structure of KDP with Sp are discussed. The Sp’s in neutral, ±1, and +2 charge states lead to the formation of an isolated SO4 in KDP and no defect state appearing in the energy gap. However, Sp in the -2 charge states induces defect states in the energy gap and therefore laser-induced damage threshold will be reduced.