The Al-40Si alloy modified by 1.0 wt% Sr addition was heat-treated using L9(34) orthogonal test. The mechanical properties were measured. The microstructures were analyzed using optical microscope. After heat treatment, the hardness decreases about one times. The eutectic matrix and primary Si phase are both refined. The primary Si phase is refined and homogeneously distributes in the matrix with the solution temperature and time increasing. The optimal heat treatment condition is that the solution temperature, solution time, aging temperature, and aging time are 550 °C, 18 hours, 200 °C, and 5 hours, respectively. In addition, the theoretical optimal value for the hardness is 20.6 HRC, which is coherent with the practical optimal value.