Additive manufacturing (AM) is an emerging technology that has been widely used for fabricating components with highly complex shapes. In this work, C-containing FeCoCrNi high entropy alloy with full density was successfully fabricated using selective laser melting (SLM). The effects of processing parameters on the microstructures and mechanical properties were investigated, and the underlying mechanism was discussed. The density of HEA is dependent on the SLM processing parameter. The increase of the SLM scanning power or the decrease of the scanning speed leads to the increase of the density of HEA. The grain size reduced when the high scanning power or the low scanning speed was used. The C-containing HEAs with full density has a yield strength of 650 MPa, and an elongation of 13.5%.