The invention relates to a preparation method of reactive type halogen-free flame-retardant polyether glycol and belongs to the field of organic synthesis.The preparation method comprises the steps that tetramethylol phosphide is subjected to a formaldehyde removal reaction under the action of a catalyst, and then an oxidation reaction is conducted in certain solvent and at a certain temperature, so that trisphosphine oxide is prepared ; after purification is conducted, a product and epoxypropane/oxirane are subjected to an addition reaction in a certain pressure environment and a catalyst condition, so that a product, namely reactive type halogen-free flame-retardant polyether glycol, is obtained.By means of the method, raw materials are low in price and easy to obtain, the reactions are simple and controllable, the yield reaches 95% or above, the obtained reactive type halogen-free flame-retardant polyether glycol is medium in viscosity and good in compatibility with foaming agents and other auxiliary agents, a combined material compounded with the reactive type halogen-free flame-retardant polyether glycol is uniform, transparent, good in fluidity and good in stability, and certain industrial prospect is achieved.