Grain refinement can enhance the strength and ductility of Al3 Mg alloys by reducing the number of casting defects and grain size. The effects of the addition of trace amounts of Ti on the microstructure and grain refinement mechanism of the Al-5Mg alloy were studied. The results show that the addition of the trace element Ti can refine the microstructure of the Al-5Mg alloy. Compared with that of the alloy without Ti, the grain size of the Al-5Mg alloy with 0.1 wt. % Ti decreases from 143.4 μm to 42.7 μm. The main reason for Ti refining grains is that Ti and Zr atoms combine with Al atoms to form Al3 (Ti, Zr), Al3 Ti, and Al3 Zr particles, which provide heterogeneous nucleation sites for Al atoms, promoting the formation of microstructures with uniform and fine equiaxed grains. When the Ti content is 0.2 wt. %, the grain size of the alloy increases to 64.6 μm, and the agglomeration and coarsening of the Al3 Ti, Al3 Zr, and Al3 (Ti, Zr) particles lead to a reduction in the number of heterogeneous nucleation sites and the effectiveness of the grain refinement of the Ti in the Al-5Mg alloy. The grain refinement effect improves as the size of the dispersed Al3 M phases in the alloy decreases and their quantity increases.