The present invention relates to a preparation method for an environmentally-friendly anti-yellowing curing agent with a good adhesive force. The preparation method comprises the following steps of: step 1, adding TDI into a solvent for dissolving, and adding a low molecular weight alcohol containing 1-3 hydroxyl groups for a prepolymerization reaction, wherein the low molecular weight alcohol accounts for 20%-35% of the total amount of the alcohol; step 2, after the prepolymerization reaction, adding HDI and the remaining low molecular weight alcohol containing 1-3 hydroxyl groups, and simultaneously adding a silanol for the prepolymerization reaction, wherein the molar ratio of TDI : HDI = 0. 5-1:1, and the molar ratio of the salinol: HDI=0.05-0.2:1; and step 3, after the prepolymerization reaction, adding a trimerization catalyst, continuously detecting contents of NCO (NCO%) and free TDI (TDI%), and when the content of NCO (NCO%) reaches 8.5% and the content of a free monomer is less than 0.5%, adding a polymerization inhibitor to terminate the reaction, and cooling for material unloading. According to the method of the present invention, a trimerization method is used for reducing the content of the free monomer, the process flow is simple, no large investment in equipment is needed, the production cost is low, the product performance fully fulfills the application requirements in the paint field, and is conductive to environmental protection and health; and when the curing agent is used in coatings, the paint film has good adhesive force with the substrate, and the recoating adhesive force is more excellent, and anti-yellowing and anti-weathering performance of the paint film is better.