Preparation method of pirfenidone crystal form III

The invention relates to the technical field of pirfenidone bulk drug preparation, and provides a pirfenidone crystal form III preparation method, which comprises: S1, mixing pirfenidone and a solvent A, stirring, heating to a temperature of 50-85 DEG C, dissolving, and filtering; s2, the temperature of the filtrate is controlled to be 25-50 DEG C, a pirfenidone crystal form III product is added for induced crystallization, and the adding amount of the pirfenidone crystal form III product accounts for 0.1%-5% of the mass of the pirfenidone added in the step S1; s3, programmed cooling is conducted on the devitrification feed liquid, then when the devitrification feed liquid is cooled to-10 DEG C to 5 DEG C, a solvent B is added for devitrification, and the solvent B is one or more of petroleum ether, isopropyl ether, diethyl ether, isopropyl alcohol and n-heptane; and S4, filtering and drying to prepare a pirfenidone crystal form III product. According to the technical scheme, the pirfenidone crystal form III