The invention provides a preparation method of rigid polyether polyol for full-water foaming. The preparation method is a process of preparing the rigid polyether polyol for full-water foaming and rigid foam from low-polymerization degree phenolic resin and activated tripolycyanamide as a composite initiator and ethylene oxide and epoxy propylene as polymeric monomers. The polyether polyol has theadvantages as follows: 1, a full-water foaming system is low in viscosity, high in fluidity and uniform in mixing; 2, the foam is good in heat-insulating performance; 3, the dimensional stability athigh and low temperatures is high; 4, the foaming system does not use Freon, 141B and other substances having a destructive effect on the atmosphere; and 5, a certain flame retardance is achieved. Thepreparation method is applied to preparation of the rigid polyether polyol for full-water foaming.