The invention provides a precise alloy casting for a mechanical arm and a casting method of the precise alloy casting, and relates to the technical field of precise alloy materials. The precise alloy casting comprises chemical elements including, by mass percent, 1.1%-1.6% of C, 1.8%-3.5% of Mn, 1.52%-2.83% of Al, 0.11%-0.2% of Cr, 0.08%-0.16% of Ni, 0.12%-0.19% of Co, 0.23%-0.37% of W, 0.22%-0.43% of Cu, 0.12%-0.43% of Nb, 0.12%-0.20% of Fe, smaller than or equal to 0.04% of Re, smaller than or equal to 0.04% of S, smaller than or equal to 0.04% of P and the balance Ti. According to the precise alloy casting, rare metal elements are adopted, many external cores are formed when the casting is cast, grains are refined or the crystal habit is changed, and the precise alloy casting is formed; and accordingly, the mechanical performance of the casting is improved, the surface smoothness of the casting is high, internal defects of the casting are few, components are pure, and performance is stable.