The squeeze casting process of ZL104 aluminum alloy flywheel housing with large wall thickness difference and complex shape was simulated by ProCAST software. The results show that the filling process was stable and could be divided into four stages: connection of channels, filling of horizontal zone, vertical direction, and difficult-filling zone. There were six characteristic zones with solidification lag, where shrinkage cavity and shrinkage porosity were obvious. The location prediction of defects was accurate. Through range analysis of defects volume, the optimal combination of process parameters was determined as pouring temperature of 650 ℃, specific pressure of 48 MPa, mold temperature of 220 ℃, local specific pressure of 800 MPa, and pressure delay time of 10 s (Side A) and 12 s (Side B). The maximum stress occurred in the thin-walled structure with fast solidification and large curvature. The simulation results were verified by the actual process.