The invention provides a polyurethane adhesive and a preparation method thereof. The polyurethane adhesive comprises diphenylmethane diisocyanate, styrene grafted polyether polyol, epoxide resin, methyl p-toluenesulfonate, polypropylene glycol, polyoxypropylene pentaerythritol ester, an amine catalyst, isophorone diamine, zeolite, castor oil, N-ethyl-N-zinc phenyl dithiocarbamate, resorcinol, triphenylphosphine, a silane coupling agent and an emulsifying agent. The preparation method comprises the following steps of: firstly mixing the diphenylmethane diisocyanate, the amine catalyst, the epoxide resin, the methyl p-toluenesulfonate, the polypropylene glycol, the polyoxypropylene pentaerythritol ester and the zeolite, and then adding the resorcinol and the triphenylphosphine; stirring, and then adding a mixture of the styrene grafted polyether polyol, the isophorone diamine, the castor oil, the N-ethyl-N-zinc phenyl dithiocarbamate, the silane coupling agent and the emulsifying agent to obtain the polyurethane adhesive. The polyurethane adhesive provided by the invention has the advantages of excellent yellowing resistance, ageing resistance, heat resistant property, hydrolysis resistant property and bonding strength.