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A series of new trifluoromethyl‐substituted phenyl alkyl ketoxime esters of Bispyribac were synthesized as potential herbicides. All synthesized compounds were characterized by nuclear magnetic resonance (NMR) spectroscopy and high‐resolution mass spectrometry (HRMS). Sprout method and foliar spray method were used to study the herbicidal activity of the synthesized compounds. Among all synthesized compounds, compound 3 (4‐trifluoromethylphenyl n‐butyl ketoxime ester of Bispyribac) exhibits the highest herbicidal activity against Echinochloa crusgalli (L.) Beauv with median effect dose (ED50) of 1.7 μg/mL in pre‐emergence herbicidal activity study. The structure‐activity relationship analysis indicates that trifluoromethyl substitution on the phenyl group exhibits better activity than methoxy and methyl substitution on the phenyl group. The alkyl chain length also affects the activity significantly and the optimal alkyl chain length is four among the tested compounds. However, the substitution position of trifluoromethyl exhibits very limited influence on the activity. In post‐emergence herbicidal activity study, compound 3 (ED90 69.1056 g/hm2) exhibits better herbicidal activity against Echinochloa crusgalli (L.) Beauv than Pyribenzoxim (ED90 99.0810 g/hm2). Moreover, compound 3 exhibits better safety to hybrid rice variety II you 084 and rice variety Nanjing 44 than Pyribenzoxim in crop safety study. Compound 3 may serve as a lead compound for future herbicide discovery. [ABSTRACT FROM AUTHOR]