Foxtail millet (Setaria italica (L.) P. Beauv.) is a significant minor crop in China, having the characteristics of drought resistance and infertile soil tolerance, high water use efficiency, wide adaptability, and richly nutritious grains. However, the weeding problem has been restricting intensive cultivation and promotion and development of organic dry farming industry of foxtail millet. In order to quickly select herbicide-resistant foxtail millet varieties, the M2 population of mutants of foxtail millet ‘Jingu 21’ was used as the test materials. The initial selection and re-screening of the ‘Jingu 21’ mutants population were carried out by spraying 0.33% sethoxydim herbicide in order to quickly obtain mutant plants and identify their molecular characteristics. The results showed that 39 mutant plants resistant to sethoxydim were preliminarily selected in the initial screening of 0.33% sethoxydim sprayed at the seedling stage in the field. From these 39 mutant plants, 9 mutants were identified after respraying 0.33% sethoxydim. A total of 15 plants of 3 mutant strains were molecularly identified, and it was found that the 1780th amino acid encoded by the ACCase gene was mutated from isoleucine to leucine in 4 mutant plants, while no change was found in other plants. The results would provide research materials for improvement of the elite millet variety ‘Jingu 21’ and accelerate breeding of herbicide-resistant millet varieties.