In this paper, a significantly photoinduced synergy between ammonium nitrate and sodium sulfite via dye decolorization was first found. This study mainly aims to explore the influences of several fundamental aspects on the photoinduced synergy as well as discuss the detailed mechanisms. The dye removal efficiencies of methyl orange and methylene blue of the synergistic system are much higher than that of a single one, and they reach 96.4% and 90.7% when the illumination is 6 and 14 min, respectively. The optimum mass ratio of sodium sulfite and ammonium nitrate in the reaction system is 1:1. The reaction process of photoinduced synergy follows the first-order reaction equation. Effects of different structures of dyes, amount of sodium sulfite and initial dye concentration on the synergistic effect were investigated. The changes of UV-vis spectra in the course of photoinduced synergy were also examined. The excellent synergistic effect can owe to the simultaneous photoreduction and photooxidation reaction with respect to photoinduced hydrated electrons (eaq−) and SO4•− active species, respectively. This work may provide some insight into detoxifying water contaminants in practical applications as well as developing other novel photoinduced synergistic systems with high performance.