An FeCrNiCoMnB_x high-entropy alloy coating was prepared by laser cladding on a low carbon steel substrate. The effect of boron addition on the microstructure, hardness, and wear resistance of FeCrNiCoMnB_x was studied. The formation mechanism of stacking faults (SFs) in boride was analyzed. Results showed that the coatings had a simple FCC structure with boride precipitation. Boride precipitation was mainly the (Cr, Fe)₂B phase when the boron content x was increased from 0.25 to 0.75. However, as the value of x reached 1.0, many (Fe, Cr)₂B phases were found in the boride. The volume fraction of borides increased with the increase in boron content. Hardness and wear resistance of the coating were also enhanced with the increase in boron content. (Fe, Cr)₂B borides had high-density SFs on the (10) planes. The generation of SFs in (Fe, Cr)₂B occurred probably because of the (Fe, Cr)₂B → (Cr, Fe)₂B phase transformation generated by a simple shear of (Fe, Cr)₂B (10) planes (with shear vectors at nearly 1/4 [111]).