The invention discloses a synthetic method of a high-activity rigid foam polyether polyol. In the method, a mixed initiator is polymerized with an epoxy compound at the pressure of 0-0.5Mpa and the temperature of 50-150 DEG C by taking an amine compound as a catalyst to obtain the polyether polyol; the mixed initiator consists of a polyol compound and a urea compound or consists of the polyol compound, a polyamine compound and the urea compound, and the urea compound accounts for 30-70% of the total weight of the mixed initiator in the mixed initiator. The polyether polyol synthesized by the method is characterized by high activity, low viscosity and low cost, can be used for the preparation of rigid polyurethane foam, and is especially applicable to the preparation of spraying polyurethane.