Lightweight aluminum alloy conductor materials (Al-Mg-Si alloys) require not only high electrical conductivity to reduce electrical loss, but also high strength to withstand extreme weather conditions. To improve electrical conductivity and mechanical properties of Al-Mg-Si alloy simultaneously, the rare earth La was introduced to modify the Al-Mg-Si alloy. The effect of La addition on the microstructure, tensile properties and electrical conductivity of cast Al-Mg-Si alloy was investigated systematically. Results indicate that the appropriate La content is helpful to improve the strength and electrical conductivity of Al-Mg-Si alloys. When the addition of La is 0.2wt.%, the α-Al grains are refined apparently, Mg and Si solute atoms in the Al matrix are reduced by the formation of Mg2Si phase; the distribution of Al11La3 phases is uniform, and the morphology of AlFeSi phase transforms from continuous state to discontinuous state. The Al-Mg-Si-0.2La alloy exhibits the optimal tensile properties and electrical conductivity, with an ultimate tensile strength of 170 MPa, a yield strength of 88 MPa, an elongation of 18.9%, and an electrical conductivity of 44.0% IACS. These values represent improvements of 9.0%, 15.8%, 70.3%, and 17.3%, respectively, compared to the Al-Mg-Si alloy without La addition. However, excessive La deteriorates the properties of Al-Mg-Si-xLa alloys.