The invention provides a preparation method of 2-cyanophenothiazine. First, a certain amount of industrial-grade 2-chlorophenothiazine, cuprous cyanide, N-methylpyrrolidone, and potassium iodide are added to the reactor, and the temperature is raised to reflux reaction. . The present invention is characterized in that, after the reaction is completed, cool and add a certain amount of water to force out the product and inorganic impurities, filter and dry to obtain a solid mixture, and extract the crude product of 2-cyanophenothiazine with ethyl acetate; 2- The crude product of cyanophenothiazine was dissolved in N, N 1 -In dimethylformamide, add an appropriate amount of phosphorus oxychloride at room temperature for dehydration reaction; then add an appropriate amount of inorganic weak alkali aqueous solution to precipitate the product, and the aqueous solution is weakly alkaline; filter, wash with water, dry, and recrystallize through toluene, The product purity is greater than 99%, the content of impurity amides is less than 0.15%, and the total yield is greater than 85%.