Large-scale applications of magnesium alloys are limited because of their poor corrosion resistance. Ce is the most common rare earth element and has the advantages of large reserves, low density, and good economic benefits, playing a significant role in improving the corrosion resistance of magnesium alloys. The effect of adding 1 wt.% Ce to the as-cast microstructure and corrosion behavior of the AT61 alloy was investigated. According to the XRD and SEM/EDS results, the Mg17 Al12 phase in the AT61 alloy transforms from a continuous distribution along the grain boundaries to a discontinuous distribution with a rod-like or block shape after the addition of 1 wt.% Ce, and the quantity significantly decreases. In addition to the Mg17 Al12 and Mg2 Sn phases, new needle-like Al4 Ce phases are present in the AT61-Ce alloy. According to hydrogen evolution tests and electrochemical performance tests, the corrosion resistance of the AT61 alloy is significantly improved. The self-corrosion current density and polarization resistance of the AT61-Ce alloy are 14.223 μA/cm2 and 2 279.951 Ω/cm2, respectively. The excellent corrosion resistance of the AT61-Ce alloy is attributed to the decrease in the volume fraction of the Mg17 Al12 phase, the weakening of galvanic corrosion, and the continuous distribution of the Al4 Ce phase along the grain boundaries hindering corrosion propagation.