A new phase-field model for dendrite growth of multiple grains is developed based on the Ginzberg-Landau theory and phase-field model of single grains. Taking Al-2mole-Cu alloy for example, in connection with the concentration field, we simulated the dendritic growth process of multiple grains during isothermal solidification. The simulation showed dendritic competitive growth of multiple grains, and reproduced the process of dendrite growth in practical solidification.