Regarding the abundant solar energy on the earth, photochemical method is the most attractive approach for the decomposition of atrazine pollutant which is widely existed in surface water. In this study, we report that the presence of sulfite can alter the photochemical atrazine degradation pathway to cleave its s-triazine ring. Density functional theory calculation results first suggested that a reductive process might be highly possible to break the s-triazine ring of atrazine by comparing the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap values of six possible atrazine degradation intermediates. The subsequent experimental results reveal that the presence of sulfite shifted a direct atrazine hydroxylated dechlorination pathway to an indirect photo-degradation route with the aid of hydrated electrons and hydrogen atoms, which were produced from the photolysis of sulfite solution under UV irradiation. This rational degradation pathway change increased the atrazine degradation and dechlorination rates by 4.3 times, and also effectively cleaved s-triazine ring, as confirmed by the generation of ammonium ions and small molecule acids during the degradation of 2,4-diamino-1,3,5-triazine with this sulfite promoted photochemical reduction process. This study provides a mild strategy to completely mineralize atrazine, and also sheds light on how the degradation pathway could affect the s-triazine ring cleavage.