Abstract: In order to explore the feasibility of using high-entropy alloys as an alternative to traditional metal materials to fabricate ship propellers, FeCoCrNiMo_x (x = 0, 0.2, 0.5, and 0.8) high-entropy alloys were fabricated in situ by plasma arc powder additive manufacturing technology, and the influence of Mo content on the microstructure and properties of the alloys was systematically investigated. The results indicate that a trace amount of Mo (x = 0.2) refines grains and increases dislocation density through lattice distortion and delayed diffusion. The alloy maintains a single-phase FCC structure. The yield strength, tensile strength, and elongation after fracture reach 384 MPa, 743 MPa, and 42.8%, respectively. Compared with the Mo-free alloy, its strength increases while the plasticity does not decrease. Excessive Mo (x ≥ 0.5) promotes the precipitation of the hard and brittle σ phase with a network distribution, resulting in a simultaneous decrease in strength and plasticity; the fracture mechanism changes from ductile fracture to brittle fracture. With the increase of Mo content, the microhardness increases from 172 HV_0.5 to 356 HV_0.5, and the average friction coefficient decreases from 0.66 to 0.47, significantly improving the wear resistance. This paper elucidates the regulatory mechanism of Mo on the microstructure and properties of high-entropy alloys, providing a theoretical basis for the component design of high-entropy alloys in additive manufacturing.