The invention discloses a preparation method of a polyurethane adhesive for an electrical device, and relates to the technical field of polyurethane adhesives. The preparation method includes the steps: adding 1, 4-butanediamine into isopropyl alcohol, adjusting pH (potential of hydrogen), dripping hydrogen cyanamide under ice-bath, stirring mixture, heating the mixture to perform reaction, addingtriethanolamine, and performing stirring, concentration and re-crystallization to obtain a material a; mixing white carbon black, carbon nano-tubes, bentonite, titanium dioxide, flame retardants andplasticizers to obtain a material b; performing dehydration on hydroxyl-terminated polybutadiene, tetrahydrofuran-epoxypropane copolyether triol and polyethylene glycol adipate glycol, adding isophorone diisocyanate, dibutyltin dilaurate and N-methyl morpholine under protection of nitrogen, and heating and stirring mixture to obtain a polyurethane prepolymer; adding diethanol amine, the material aand benzyl bichloride to perform stirring reaction, adding the material b, stirring mixture to obtain the polyurethane adhesive. The polyurethane adhesive has excellent antibacterial and mold-proof performance, electric insulation performance and heat stability and can keep good electric insulation performances at high temperature.