The invention discloses a diphosphite synthesis method. According to the diphosphite synthesis method, a tested dosage of phosphorus trichloride, 2,2'-diphenol and 3,3'-di-tert-butyl-5,5'-dimethoxy-2,2'-diphenol are used as raw materials, the phosphorus trichloride is dissolved in an organic solvent to prepare a solution, then the solution is added into a 2,2'-diphenol and organic alkali dissolved organic solution under stirring, and the reaction solution is continuously stirred for several hours after addition of the phosphorus trichloride solution is completed; a 3,3'-di-tert-butyl-5,5'-dimethoxy-2,2'-diphenol dissolved organic solution is added into the reaction solution, the reaction solution is continuously stirred for several hours after adding is completed, then heating is performed to reach the temperature of 80-120 DEG C, and reflux is performed for several hours; the reaction liquid is cooled to reach room temperature after reaction is completed, filtration and filter cake washing are performed, filtrates are mixed and then are evaporated and concentrated to obtain solid, the solid is washed with acetonitrile, and recrystallization is performed to obtain a product. The diphosphite synthesis method has the advantages of being high in diphosphite yield, small in phosphorus trichloride usage amount, few in by-products, short in reaction time, simple in aftertreatment and the like.