Numerical simulation of solidification microstructure of aluminum alloy casting is studied by using a macro-micro coupled method. Different mesh sizes and time steps are used to calculate the macro temperature field and the crystal growth. The microstructure simulation is conducted by selecting a macro cell in the central region of the aluminum casting, and then the initial distribution of the temperature in the micro domain is obtained by interpolating the temperature of the macro cells near the selected cell. The crystal growth is controlled by the phase field equation, and the mathematical model also includes mix solution conservation equation and energy conservation equation to compute the distribution of the solution and the temperature. Noise is introduced to simulate the side branches. To describe the heterogeneous nucleation the Gaussian distribution is applied. The interface undercooling is considered to be the sum of thermal, solute and curvature effects. The 2-D dendritic growth morphology, the 3-D dendritic growth morphology and the crystal sizes in the region are simulated. The simulation results are compared with those obtained by experiments.