Phosphite (HPO₃^2-, H₂PO₃^-, +3valence) is a reduced form of phosphorus in the biogeochemical phosphorus cycle. Its photooxidation process can be an important path of its transformation in water environment. The photooxidation process of 1μmol/L phosphite in nitrate solution was studied using 300W mercury lamp as light source. Experiments were carried out to study the influences of NO₃^- concentration, pH value and common ions in water (such as Cl^-, SO₄^2-, HCO₃^-, Fe³⁺, Mn²⁺) and humic acid, on the photooxidation of phosphite in NO₃^- solutions. The results demonstrate that the photooxidation of phosphite followed the pseudo-first-order kinetics and the photooxidation rate increased rapidly with increasing initial NO₃^- concentration. The kinetics constant changed from 0.020h^-1 to 0.271h^-1 with the concentration of NO₃^- increased from 0 to 120μmol/L. The kinetics constant decreased with the increase of pH and the addition of HCO₃^-. The addition of Cl^- and SO₄^2- slightly inhibited the photooxidation process. The addition of Fe³⁺ accelerated the photooxidation of phosphite in NO₃^- solution while Mn²⁺ inhibited the photooxidation. The addition of humic acid inhibited the photooxidation process. ROS (reactive oxygen species) were found to play an important role in the phosphite photooxidation reaction by using isopropanol and sodium azide (NaN₃) as ROS quenchers. In the simulated Taihu Lake water experiment, the increased amount of phosphate coincided with the decreased amount of phosphite, which indicated that the photooxidation product of phosphite is phosphate.