The invention discloses a 4H-1,3-benzothiazine derivative with antibacterial activity and a synthesis method and applications thereof. The structure of the 4H-1,3-benzothiazine derivative is represented by the formula (I), wherein in the formula (I), R1 represents a hydrogen atom, a chlorine atom, a bromine atom, or a methoxyl group, and R2 represents a hydrogen atom, a chlorine atom, a bromine atom, a methyl group, a methoxyl group, or a trifluoromethyl group. The 4H-1,3-benzothiazine derivative has a good antibacterial effect and can be used as a potential antiseptic.