A manufacturing method for an isothermal evaporation casting process is disclosed and to design the manufacturing method for the simple atmospheric casting. The present invention can melt the Mg—Ni alloy with the aspect of totally different melting points thereof, and the other elements can be added during melting simultaneously. Through the Mg—Ni alloy with a suitable weight ratio, the eutectic alloy of Mg/Mg2Ni and high purified γ-phase Mg2Ni can be made. Then adding other elements are to produce the composition of Mg/Ni/M. By way of the manufacturing method of the present invention, the melting equipment is simplified; and, through different manufacturing steps, the purposes of time-saving and low cost are reached; further that, a large amount of the eutectic alloy of Mg/Mg2Ni with different kinds of ratios of Mg, high purified γ-phase Mg2Ni, or the composition of Mg/Ni/M can be obtained.