The integration of UV/sulfite autoxidation process (USAP, i.e., UV activation of sulfite in the presence of 5 ∼ 10 mg/L O₂) into conventional water to degrade micropollutants rises extensive attention, but its impact on water quality, and especially the formation of disinfection byproducts is still unclear. Herein, the formation of dibromoacetonitrile (DBAN) from bromate (BrO₃^–) upon treatment with USAP followed by chlorination was evaluated, in the presence of amino acids (AAs) selected as representative organic matter in drinking water. Results revealed that hydrated electrons (e_aq^–) produced during USAP contribute to the reduction of BrO₃^– to Br^–, which is then converted into HBrO/BrO^– during post-chlorination. At the same time, sulfate radicals (SO₄^•–) and hydroxyl radicals (•OH) generated in USAP mediated AAs’ conversion via α-hydrogen abstraction and NH₂-hydrogen abstraction reactions to produce HN=C(CH₃)‒COOH, CH₃‒CH=NH, and CH₃‒CN, which are released into the post-chlorination stage and therefore, enhance the bromine utilization factor (BUF) value and DBAN formation. The effects of the USAP treatment time, BrO₃^– concentration, AA concentration, pH, and real waters were also evaluated. Although 63.5% of BrO₃^– was eliminated by USAP followed by chlorination, the toxicity index (TI) was increased by 1.5-fold due to the formation of the all brominated CX₃R-type nitrogenous disinfection byproducts (N-DBPs), demonstrating the potential risk of applying USAP as a treatment process in BrO₃^– containing waters.