The invention discloses an ODOPB (10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-base polyphosphate flame retardant which is characterized in that n is a whole number ranging from 2000 to 4000, and Y is a hydrogen atom or alkyl group of which the carbon atom number is less than 6. The preparation method comprises the following steps: weighing phenyldichlorophosphate or phenyl dibromophosphate, ODOPB and a catalyst, wherein the mol ratio of phenyldichlorophosphate or phenyl dibromophosphate to ODOPB is 1:(0.9-1.2), and the mol ratio of catalyst to ODOPB is 0.001-0.01; adding the ODOPB and the catalyst into a reactor, heating to 140 DEG C, adding the phenyl dichlorophosphate or phenyl dibromophosphate, keeping the temperature for 1-2 hours, heating to 200-240 DEG C, and keeping the temperature for 2-4 hours, thereby obtaining a white solid flame retardant; and adding the white flame retardant into a solvent until the white flame retardant is completely dissolved, precipitating with 5-10 times of solvent amount of precipitating agent, filtering, and carrying out vacuum drying to obtain the white solid product. The ODOPB-base polyphosphate flame retardant has favorable compatibility with the polymeric matrix, can not easily transfer, and has excellent durability.