The aim of this paper is to investigate the photochemical degradation of procymidone. The influences of 3 light sources, 3 initial concentrations, pH, nitrate and H₂O₂ on the photodegradation of procymidone in water were studied. The results showed that the photochemical degradation of procymidone in water followed the first order reaction. The photolytic rate of procymidone under UV lamp was about two times of that under high-pressure mercury lamp, and its half -life was 43.6 min under high-pressure mercury lamp and 28.2 min under UV lamp. Using high-pressure mercury lamp as light source, the photolytic rate of procymidone was negatively correlated with its initial concentration when it was at the range of 2 mg·L^-1 to 8 mg·L^-1. And the photolysis rate constant decreased with the increasing of solution pH. Nitrate had quenching effect on photolysis of procymidone in aqueous solution under HPLC (high performance liquid chromatography). And H₂O₂ had photosensitization on the degradation of procymidone.