A preparation method for a reaction-type halogen-containing flame-retardant polyether polyol belongs to the field of organic synthesis. The preparation method comprises: initiating a ring-opening polymerization reaction of chloropropylene oxide by a bromine-containing dihydric alcohol/phenol initiator under the effect of a catalyst, to prepare a bifunctionalpolyepichlorohydrin; reacting bifunctionalpolyepichlorohydrin with a strong alkali solution to remove partial chlorine atoms and further to obtain an epoxy-terminated compound; under the effect of an acidic catalyst, in the presence of water and an organic solvent, performing a hydrolysis reaction on the epoxy-terminated compound to obtain a multi-functional halogen-containing flame-retardant polyether polyol solution; performing reduced pressure distillation on the polyether polyol solution to obtain the product halogen-containing flame-retardant polyether polyol. The method has the advantages of cheap and easily available raw materials, a simple and controllable reaction and a yield of 95% or more; the prepared reaction-type halogen-containing flame-retardant polyether polyol is moderate in viscosity, and good in compatibility with foaming agents or other auxiliary agents; a combined material by using the polyether polyol is uniform, transparent, excellent in fluidity and good in stability.