The invention discloses a preparation method of DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) derivatives and relates to flame retardants in the field of chemical materials. A compound and a compound B carry out a dehydration reaction for 4-30h at 1-6atm and 150-220 DEG C in the presence of a catalyst, an entrainer and a solvent to prepare a compound I, and the molecular formula of the compound I is shown in the specification. The product prepared by the preparation method of the invention is easy to purify and is high in yield and low in cost.