A new phase-field model is developed based on the Ginzberg-Landau theory, which is consistent with WBM model and KKS model. The dendritic growth process during the solidification in a Al-65%Cu binary alloy was simulated using the phase-field model. Solute diffusion equation was solved simultaneously. The effects of undercooling on the dendritic growth and solute profile in isothermal solidification of binary alloy were investigated. The results indicate that the dendritic has well-developed secondary arms with the increase of undercooling. Correspondently, the solute Peclet number and the tip speed increases, the tip radius decreases, and the solute segregation in solid-liquid interface increases. The results agree well with Ivantsov theory.