The method enhances a hardening reactivity of melamine hardener and isocyanate in the preparation of polyester. The polyester comprises 55-100 mol% of dibasic acid and its deriv.; 0-45 mol% of aliphatic or aliphatic cyclic dibasic acid or deriv.; 0-2 wt.% of above 2 types of glycols and trifunctional monomer; 0.01-2 wt.% of potassium slufonate compound A of formula (I); and 0.005-0.03 wt.% of calcium acetate. The setting activity is enhanced by adding the above potassium sulfonate compound A and calcium acetate at 180-200 deg.C during the manufacture process. This polyester is used as binder resin of paint, ink and coating.