Abstract: The low-silicon cast aluminum alloys containing different mass fractions (0, 0.03%, 0.06%, 0.09%, 0.12%, 0.15%) of Y were prepared and treated by T6 heat treatment. The effect of Y addition amount on the microstructure and tensile properties of the alloy were studied, and the mechanism was analyzed. The results show that with the increase of Y addition, the α-Al nucleation temperature and eutectic silicon transformation temperature of the alloy first increased and then decreased, the α-Al and eutectic silicon in the alloy were first refined and then coarsened, the as-cast morphology of eutectic silicon changed to threadiness, and the heat treated state morphology of eutectic silicon changed to spherutic, the yield strength and tensile strength first increased and then decreased, and the percentage elongation after fracture basically decreased. When Y mass fraction was 0.09%, the α-Al nucleation temperature and eutectic silicon transformation temperature were the highest. After heat treatment, the microstructure was fine α-Al dendrite and spherutic eutectic silicon, the yield strength, tensile strength and percentage elongation after fracture were relatively large, namely the comprehensive mechanical properties were the best. When excessive Y was added (0.15%), the coarse acicular Al₂Si₂Y phase was precipitated in the alloy, and the tensile fracture mode changed from the tough-brittle mixed fracture to the brittle fracture.