The phase-field model based on Kim model which couples the solute field is used to simulate microstructural dendrite growth of multiple grains for Al-2-mole-Cu alloy. In order to improve the efficiency of calculation, an interpolative method is used to achieve the coupling of the macro field with the micro field. The influence of different undercooling on shapes of dendrites and distribution of solute is studied.The result shows that the re-distribution of solute can decrease the melting point of the alloy, and reduce the practical undercooling, so the constitutional supercooling can influence the growth rate and solute distribution of dendrites by decreasing the freezing point evidently, and at the same time results in the cease of growth for both collided dendrites and perferred growth in directions of unhindered grains.