The invention discloses a method for coproducing BIT (1,2-benzisothiazoline-3-ketone) and MBIT (2-methyl-1,2-benzisothiazoline-3-ketone). The method comprises the following steps: 1) performing hydrolytic cyclization reaction on o-methylthiocyanophenyl serving as a raw material, water and chlorine, filtering and salifying the reaction liquid after the reaction to obtain a BIT salt aqueous solution, acidizing and filtering to obtain a BIT product, and absorbing, pressurizing and condensing a byproduct methane chloride generated in the reaction process to collect for later use; 2) adding the BIT salt aqueous solution obtained in the step 1) into a high-pressure kettle, adding the byproduct methane chloride obtained in the step 1), and performing temperature-increasing reaction to obtain a mixture containing the MBIT and a byproduct MOBIT (3-methoxyl-1,2-benzisothiazoline); 3) adding the mixture containing the MBIT and the byproduct MOBIT and obtained in the step 2) into the hydrolytic cyclization reaction in the step 1), performing demethylating reaction on the byproduct MOBIT to change into BIT and methane chloride, combining the MBIT with hydrogen chloride to form MBIT hydrochloride, and performing separation steps such as filtering and neutralizing on the reaction mixture to obtain BIT and MBIT products separately. The method has the advantages of reasonable process, energy saving, low pollution, capability of greatly reducing production cost and the like.