The reasons and the prevention methods for common defects during the lost foam process of Al alloy have been analyzed, such as casting deformation, cold shut, misrun, cockle, porosity, pinhole, hydrogen gas pick-up, oxidation, slag inclusion, shrinkage, grain coarsening, poor air tightness, sinking shape, filling-shortage etc. These above problems are explored from different angles to orientate processes with foam pattern, coating, dry sand, aluminum alloy melting, pouring etc.