To improve the ductility of a commercial Mg-rare earth alloy EV31A (Mg-3Nd-1.5Gd-0.3Zn-0.5Zr), a heat treatment method called double aging is explored, and its effect on mechanical properties and microstructure of the alloy is studied. Ultimate strength and elongation of the alloy can be increased to 288 MPa and 6.6% by the optimum double aging process, compared to 273 MPa and 4.9% after single aging. Time consumption of the aging process is also significantly decreased from 16 h (single aging) to 2 h. HAADF-STEM characterization shows that the primary precipitate is β'''' phase, which is similar to β'''' phase in Mg-Nd binary alloy. By double aging, the β'''' phase is finer and more densely distributed compared with single aging, with approximately double density and half size, which explains the improvement in strength and ductility.