The present invention discloses a preparation method of high-functionality and high-flame-retardance polyol. The method comprises the following steps: mixing bromine-containing epoxy resin with dihydric alcohol with a mole ratio of 1: (1.9-3.0), adding a polyurethane catalyst accounting for 0.05%-0.65% of the total mass of raw materials, carrying out heating to 120-240 DEG C for reaction for 1.5-6.5 hours; removing unreacted dihydric alcohol from the obtained mixture under a vacuum condition at 210-235 DEG C; and adding an intermediate product with dihydric alcohol removed into a phosphorus-containing flame retardant with a mass ratio of 15-25%, and carrying out uniform stirring at the temperature of 120 DEG C to obtain the product. The advantages of the method disclosed by the present invention are that: on the basis of common raw materials in the market, a bromine-containing polyether polyol product with high functionality, high strength, and high flame-retardance can be produced with lowered production conditions.