The invention relates to the synthesis technique improvement of disperse red 91 which uses the disperse red 60 and hexanediol as raw materials, uses potassium carbonate or potassium hydroxide as an acid binding agent and uses an emulsifier of OP series as a catalyst; after the raw materials are condensed at high temperature to the terminal, the product can be segregated by methanol (or ethanol) when the temperature is cooled, and at last the product is filtrated and separated. The mother liquid is rectified to recycle the methanol (or ethanol), phenol and hexanediol, and the technique improves the product quality.