The invention discloses a preparation method for polyurethane dispersant. At first, anchorage group A is composed under lower temperature. The anchorage group is constituted by polyisocyanate and diol or amine or phenol. Polyurethane pre-polymer with two segments of isocyanate root block is composed. Then water polymer root block is used. Two ends of the water polymer are active hydroxyl groups or on two ends of the water polymer there are active hydroxyl groups. Under higher temperature and catalyst condition, polyurethane dispersant with BAB segment-embedded type is composed. The polyurethane dispersant can wet grain quickly and fully. Grinding time for achieving qualified grain fineness is shortened. The content of solid grain in grinding material can be improved greatly. Processing equipment and energy consumption can be saved. Dispersion is uniform and stability is good. The final using performance of dispersion system is improved obviously.