Aluminium-silicon alloys are widely used in various industries because of their good mechanical properties and high temperature resistance. However, the presence of coarse and irregular primary crystalline silicon phases in the original as-cast microstructure seriously affects the properties of per-eutectic aluminium-silicon alloys. In this paper, the effect of La and Ce on the microstructure and mechanical properties of A390 alloys was studied by OM, SEM and a universal material testing machine. The results show that mixed rare earth elements have an obvious metamorphic effect on primary silicon. With increasing amounts of mixed rare earth elements, the volume fraction of primary silicon and the mechanical properties of the alloy first increase and then decrease. When the amount of mixed rare earth is 0.58 wt. % (La is 0.33 wt. %, Ce is 0.25 wt. %), the volume fraction of primary silicon reaches the minimum value of 9.37%, and the tensile strength and elongation of the as-cast alloy are 229 MPa and 1.7%, respectively.