The solidification microstructure of Al-Si alloy was observed in the experiment, the second dendrite arm spacing (SDAS) was measured, and the effect of temperature on the microstructure was analyzed. Phase-field (PF) model incorporating natural convection caused by gravity was employed to simulate the microstructure evolution of Al-Si alloy under the experimental conditions. Good agreements between the experimental and simulation results verified the reliability of the simulation approach proposed in this study. Based on the proposed model, a series of simulation cases (2D and 3D) were performed to investigate the evolution of columnar and equiaxed dendritic structures. It was found that the solute content of the alloy had little impact on the microstructure evolution, while the solute expansion coefficient had obvious effect on the dendrite tip velocities. Significant improvement of computational efficiency was achieved via novel algorithms, making it possible to perform massive simulation for studying the evolution of solidification microstructures, which is hard to be directly observed in experiments via synchrotron radiation for Al-Si alloy.