A phase-field approach which incorporates both heat and solute equations for simulation of Al-Cu binary alloy solidification is studied. The dependence of dendritic growth behavior on intentionally varied thermal diffusivity Dris investigated. The calculated results indicate that with the decrease of the thermal diffusivity DT, the influence of the release of latent heat on the original temperature field decreases, the undercoolins in The melt can maintain relatively steady; while the dendritic lip velocity increases, the dendritic morphology presents more developed side-branches, and the solute segregation is small accordingly. The simulation results are compared with those obtained experimentally; they agree well with each other.