The invention relates to a benzothiazine-4-one compound containing a 2,8-diazaspiro[4.5]decane fragment and a preparation method thereof, and especially relates to a 6-trifluoromethyl-8-nitro-4H-benzo[e][1,3]thiazine-4-one compound whose 2-position substituent is 2,8-diazaspiro[4.5]decane-8-yl. Z represents alkoxycarbonyl(C1-C6), phenoxycarbonyl and benzyloxycarbonyl; or, the Z represents phenyl,pyridyl, naphthyl, quinolyl, pyrazinyl, pyrimidinyl, pyrazolyl, imidazolyl, furyl or thienyl; hydrogen atoms in any position of these groups can optionally be replaced by R groups; and R is selected from alkyl having 1-4 carbon atoms, alkoxy having 1-3 carbon atoms, halogen, -CF3, -OCF3, -NO2 or -CN. The structure of the compound is shown as formula (I).