The present invention is related to a polyurethane dispersant and a preparation method thereof. The polyurethane dispersant is terminated by a nitrogen-containing heterocyclic compound during synthesis. The nitrogen-containing heterocyclic compound is prepared by subjecting anhydride and a compound containing a primary amine and a secondary amine or a hydroxyl or mercapto group to dehydration condensation. The condensed product is the nitrogen-containing heterocyclic compound, containing a reactive hydrogen. The structural formula of the compound containing the primary amine and the secondary amine or hydroxyl or mercapto group is shown as follows: ##STR00001##
wherein n=1 to 5, and preferably 2 to 4, R.sup.1 and R.sup.2 are respectively alkyls containing 1 to 8 carbons, R.sup.1 and R.sup.2 are independent of each other or connected by a chemical bond to form one or more saturated or unsaturated 4-member to 8-member rings, and X represents N, O, or S.