A method for preparing a reactive sealant resin, comprising: preparation of a polyether polyol: under the action of an alkali catalyst, polymerizing a hydroxyl-containing initiator with an epoxide to obtain a first polyether polyol; polyether modification: adding an alkoxidation reagent and a halogenated end-capping agent containing a double bond to the first polyether polyol obtained in step for reaction to obtain a crude product of double-bond-terminated polyether, and refining the obtained crude product to obtain a product of modified polyether; and silane end-capping: subjecting the modified polyether obtained in step as a raw material and hydrogen-containing silane to a silane end-capping reaction under the action of a hydrosilylation catalyst to obtain a target product of a reactive sealant resin; the alkali catalyst is one or more of an alkali metal, an alkali metal hydride, an alkali metal hydroxide, an alkali metal alkoxide, a composite metal cyanide or a phosphazene catalyst; wherein the hydrosilylation catalyst is a supported metal platinum catalyst with an amount of 0.1 ppm to 50 ppm, based on the platinum content; the supported metal platinum catalyst is obtained by subjecting a chloroplatinic acid solution to impregnation, reduction and drying with a polyurethane flexible foam as a carrier; the polyurethane flexible foam is prepared by subjecting a raw material comprising a second polyether polyol to a foaming reaction, wherein a polymerized monomer for preparing the second polyether polyol comprises an epoxide containing a C=C double bond in the molecule, and the content of the epoxide in the polymerized monomer is 1 wt% to 40 wt%. preparation of a polyether polyol: under the action of an alkali catalyst, polymerizing a hydroxyl-containing initiator with an epoxide to obtain a first polyether polyol; polyether modification: adding an alkoxidation reagent and a halogenated end-capping agent containing a double bond to the first polyether polyol obtained in step for reaction to obtain a crude product of double-bond-terminated polyether, and refining the obtained crude product to obtain a product of modified polyether; and silane end-capping: subjecting the modified polyether obtained in step as a raw material and hydrogen-containing silane to a silane end-capping reaction under the action of a hydrosilylation catalyst to obtain a target product of a reactive sealant resin; the alkali catalyst is one or more of an alkali metal, an alkali metal hydride, an alkali metal hydroxide, an alkali metal alkoxide, a composite metal cyanide or a phosphazene catalyst; wherein the hydrosilylation catalyst is a supported metal platinum catalyst with an amount of 0.1 ppm to 50 ppm, based on the platinum content; the supported metal platinum catalyst is obtained by subjecting a chloroplatinic acid solution to impregnation, reduction and drying with a polyurethane flexible foam as a carrier; the polyurethane flexible foam is prepared by subjecting a raw material comprising a second polyether polyol to a foaming reaction, wherein a polymerized monomer for preparing the second polyether polyol comprises an epoxide containing a C=C double bond in the molecule, and the content of the epoxide in the polymerized monomer is 1 wt% to 40 wt%.