To improve the internal quality and mechanical properties of ADC12 aluminum alloy engine blocks, this study optimizes the die-casting process and T6 heat treatment via numerical simulation and experimental analysis. A spot cooling water system was integrated into the thick-walled mold regions to control solidification at a 660 °C pouring temperature. Results indicate that optimized spot cooling significantly reduced air entrainment and shrinkage porosity to 3,237.06 mm³ and 503.12 mm³, respectively, satisfying heat treatment requirements. Microstructural analysis reveals that solution treatment (520 °C, 3 h) facilitates the complete dissolution of the Al2Cu phase into the α-Al matrix. After aging at 180 °C for 8 h, the tensile strength and yield strength reached 312.8 MPa and 239.6 MPa—increases of 40.2% and 58.5% over the as-cast state. This optimal 520 °C × 3 h + 180 °C × 8 h process ensures high mechanical performance and dimensional stability while significantly shortening the production cycle compared to longer aging durations.