Abstract: (FeCoCrNi)_88-xMo₈WNb₃C_x (x=0.25, 0.5, 0.75, 1, 1.5, 2, 2.5) high entropy alloy specimens are prepared by mechanical alloying and additive manufacturing. The formation law of the high entropy alloys is clarified firstly, and then the effects of C content on their microstructures and mechanical properties are further studied. It is found out that the (FeCoCrNi) _88-xMo₈WNb₃C_x high entropy alloy is composed of FCC and M₆C phases when the C content is between 0.25% and 2.50% (mole fraction). With the increase of C content, the compressive strength of the alloy increases and the fracture strain increases at first and then decreases. When the C content is 2.00%, the compressive strength and fracture strain of the alloy are 1 993.4 MPa and 31.5%.