The impact of Mo on the microstructure, phase constitution, and tensile properties of Al_1.25CoCrFeNi_3−xMo_x (x=0.05, 0.1, 0.2, 0.3, and 0.5) high-entropy alloys (HEAs) was explored systematically through phase diagram simulation and experimental validation. The findings indicate that Mo addition transforms the microstructure from eutectic to hypereutectic and eventually to dendritic. Mo promotes the nucleation of the body-centered cubic phase by reducing the nucleation barrier and altering the valence electron concentration. As Mo content increases, yield strength rises, while the tensile strength and plasticity increase first and then decrease. Notably, the Al_1.25CoCrFeNi_2.8Mo_0.2 HEA achieves an impressive tensile strength of 1234.80 MPa and a fracture strain of 19.33%. Key strengthening mechanisms include solid solution strengthening, grain boundary strengthening, and heterogeneous interface strengthening.