Abstract: The effect of W element on the microstructure evolution and mechanical properties of Al_1.25CoCrFeNi₃ eutectic high-entropy alloy and Al_1.25CoCrFeNi_3−xW_x (x = 0, 0.05, 0.1, 0.3, and 0.5; atomic ratio) high-entropy alloys (HEAs) were explored. Results show that the Al_1.25CoCrFeNi_3−xW_x HEAs are composed of face-centered cubic and body-centered cubic (BCC) phases. As W content increases, the microstructure changes from eutectic to dendritic. The addition of W lowers the nucleation barrier of the BCC phase, decreases the valence electron concentration of the HEAs, and replaces Al in the BCC phase, thus facilitating the nucleation of the BCC phase. Tensile results show that the addition of W greatly improves the mechanical properties, and solid-solution, heterogeneous-interface, and second-phase strengthening are the main strengthening mechanisms. The yield strength, tensile strength, and elongation of the Al_1.25CoCrFeNi_2.95W_0.05 HEA are 601.44 MPa, 1132.26 MPa, and 15.94%, respectively, realizing a balance between strength and plasticity. The fracture mode of the Al_1.25CoCrFeNi_3−xW_x HEAs is ductile–brittle mixed fracture, and the crack propagates and initiates in the BCC phase. The eutectic lamellar structure impedes crack propagation and maintains plasticity.