The invention relates to the technical field of fine chemicals and discloses a novel method for preparing a medical active compound N-substituted benzothiazine-4-one. The novel method comprises the following specific steps: taking N-substituted-2-methylthiobenzamide as a raw material, and silver oxide and 1-chloromethyl-4-fluoro-1,4-diazabicyclo[2.2.2]octane bis(tetrafluoroborate) as additives, and reacting in 1,2-dichloroethane for 12 h at 140 DEG C, so as to obtain the N-substituted benzothiazine-4-one. Compared with an existing method, the method is rapid and simple and novel in reaction; an additional oxidization step is not needed; the N-substituted-2-methylthiobenzamide is used as the raw material to construct a target product N-substituted benzothiazine-4-one in one step in a high-efficiency manner; the novel method has potential industrial value and is worthy of being popularized and applied.