In order to develop a novel herbicide containing the 2,1-benzothiazine motif, a series of 3-acetyl-4-hydroxy-2,1-benzothiazine derivatives was synthesized. All the target compounds were confirmed by 1 H NMR, 13 C NMR, and high-resolution mass spectrometry(HRMS). In addition, the crystal structure of compound T27 was determined by single crystal X-ray diffraction. The bioassay results showed that some of the 3-acetyl-4-hydroxy-2,1-benzothiazine derivatives(T13, T15, T22, and T24) showed good herbicidal activity at a dosage of 100 μg/mL. Among them, compounds T22 and T24 showed promising post-emergent herbicidal activities against Brassica campestris and Amaranthus retroflexus even at a dosage of 375 g/ha(1 ha=10 4 m 2 ) in the greenhouse test. Studies on the structure-activity relationship demonstrated that the type of acetyl group played an important role in the herbicidal activity, and that the introduction of a phenoxyacetyl group at the 3-position of 2,1-benzothiazine was beneficial in improving the herbicidal activity. The present study also indicated that 3-phenoxyacetyl-4-hydroxy-2,1-benzothiazine could be a potential lead compound for further development of novel 2,1-benzothiazine-containing herbicides.