High-intensity light (HL) greatly induces the accumulation of anthocyanin, a fundamental compound in photoprotection and antioxidation. Many mechanisms regulating anthocyanin biosynthesis are well-characterized across developmental and environmental conditions; however, post-transcriptional regulation of its biosynthesis remains unclear. RNA splicing is one mechanism of post-transcriptional control and reprogramming in response to different developmental cues and stress conditions. TheArabidopsissplicing modulator SR45 regulates a number of developmental and environmental stress responses. Here, we investigated the role of SR45 and its isoforms in HL-induced anthocyanin accumulation. We found that theSR45promoter contains light-responsivecis-elements, and that light stress significantly increasesSR45expression. Furthermore, we found that mutant plants lacking SR45 function (sr45) accumulate significantly more anthocyanin under HL.SR45is alternatively spliced to produce two proteins, SR45.1 and SR45.2, which differ by seven amino acids. Intriguingly, these isoforms exhibited distinct functions, with only SR45.1 reversing anthocyanin accumulation in thesr45plants. We also identified possible SR45 target genes that are involved in anthocyanin synthesis. Consistent with the antioxidant role of anthocyanin, we found thatsr45mutants andSR45.2overexpression lines accumulate anthocyanin and better tolerate paraquat which induces oxidative stress. Collectively, our results reveal that theArabidopsissplicing regulator SR45 inhibits anthocyanin accumulation under HL, which may negatively affect oxidative stress tolerance. This study illuminates splicing-level regulation of anthocyanin production in response to light stress and offers a possible target for genetic modification to increase plant stress tolerance.