With the increasing promotion of simplified rapeseed cultivation in recent years, the development of cul-tivars with high resistance to herbicides is urgently needed. We previously developed M342, which shows sulfonylurea herbicide resistance, by targeting acetohydroxyacid synthase (AHAS), a key enzyme in branched-chain amino acid synthesis. In the present study, we used a progeny line derived from M342 for an additional round of ethyl methane sulfonate mutagenesis, yielding the novel mutant DS3, which harbored two mutations in AHAS genes and showed high sulfonylurea resistance. One mutation was the substitution Trp574Leu, as in M342, according to Arabidopsis protein sequencing. The other site was a newly recognized substitution, Pro197Leu. A KASP marker targeting Pro197Leu was developed and reliably predicted the response to sulfonylurea herbicides in the F2 population. The combination of Trp574Leu and Pro197Leu in DS3 produced a synergistic effect that greatly increased herbicide resis-tance. Analysis of the protein structures of AHAS1 and AHAS3 in wild-type and single-gene mutant plants revealed three-dimensional protein conformational changes that could account for differences in herbi-cide resistance characteristics including toxicity tolerance, AHAS enzyme activity, and AHAS gene expression.