Abstract: Mesosulfuron-methyl is a widely used herbicide for the control of Alopecurus aequalis and some other grass weeds in wheat fields. However, some A. aequalis populations have evolved resistance to mesosulfuron-methyl in Shandong, Jiangsu and Anhui Province, China. The ALS gene mutation, which occurred at 197 codon, is an important mechanism causing mesosulfuron-methyl resistance in A. aequalis. Basing on the different base sequences between the wild and mutant type, a derived c1eaved amplified polymorphic sequences (dCAPS) method was designed for rapid detection of the ALS gene mutation at 197 codon. A mismatched base was introduced at 3′-end of the primer D197F. Then the ALS fragments amplified from different A. aequalis plants showed polymorphisms after digested by the restriction enzyme BamH I. The wild plants ultimately produced two fragments (36 bp and 200 bp). The homozygous mutant plants couldn’t be digested and only produced a 236 bp fragment. While the heterozygous mutant plants produced three aforementioned fragments at the same time. The results obtained using the dCAPS method were accurate and reliable, which were consistent with the classical whole-plant experiments. Moreover, it could detect any mutations at ALS 197 codon. This study provides a theoretical basis for the rapid detection of target-site resistance to mesosulfuron-methyl in A. aequalis and other grass weeds.