The invention discloses a polymeric 
phosphorus-containing 
flame retardant with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-
oxide as a lateral group and a preparation method thereof. The preparation method is characterized by comprising the following steps: placing aromatic dihydric 
phenol and 
pentaerythritol diphosphate diphosphoryl 
chloride (or 
bromine) which are modified by the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-
oxide in an 
anhydrous acetonitrile solvent based on reaction mol ratio, heating the obtained mixture to 80 DEG C in an 
inert atmosphere, agitating and reacting until no 
halogen hydride is released, cooling the obtained mixture to the 
room temperature, and depressurizing the mixture to remove the 
solvent. The obtained product has 
high phosphorus content, very good expansibility and 
char formation, and good 
flame retardation effect. When 
flame retardation is performed on 
epoxy resin, the 
limiting oxygen index of the obtained product is greatly improved and the vertical burning scale reaches V-0 level; and the polymeric 
phosphorus-containing flame retardant has the 
advantage of overcoming the disadvantages of low 
thermal decomposition temperature, poor compatibility with 
polymer matrix and easy migration in the existing micromolecular 
phosphorus flame retardants. The preparation method has the advantages of being simple and feasible, using no catalyst and simple post-treatment process.