Method for preparing didopo compound

Disclosed is a method for preparing a compound I. The method is characterized in that the compound I is prepared by means of a reaction between a ketone compound II and a chloro organic phosphine compound III in the presence of an oxygen compound. Compared with the existing synthesis method in which DiDOPO is prepared by means of a reaction between DOPO and aryl alkyl ketone catalyzed by phosphorus oxychloride and the like, the method in the present invention shortens reaction steps, effectively reduces costs and improves the operationality and safety of the process, and the purification of a product is easier and the yield is higher.