<p>The performance of cast products can be significantly compromised by entrainment defects, which have been found in various alloys. However, there are few research reports on the types, basic characteristics, and formation mechanisms of entrainment defects during the preparation stage of semi-solid slurries, which is explored in the present work. The slope slurry production method can create the presence of entrainment defects within the semi-solid A356 aluminum alloy, which include entrapped gas pores and bifilms. These can then result in gas pore, shrinkage porosity, and bifilm defects in the resulting semi-solid castings. The bifilm, characterized by curling and winding, is a surface defect; the two layers of oxide film that make up the bifilm exhibit typical wrinkling and folding features. Entrainment defects result in lower mechanical performance and consistency of mechanical properties for water-quenched samples of semi-solid slurries prepared with inclination angles of 30° and 45°. Numerical simulation and water simulation all indicate that the slope method can induce surface turbulence, which is the intrinsic cause of the formation of bifilm and gas pore. The bifilm within the semi-solid slurry can replace the nucleation stage of shrinkage porosity. Under the influence of triaxial hydrostatic tensile stress and H diffusion during the solidification stage, the bifilm unfolds and evolves into shrinkage porosity.</p>