A series of WC‐based cemented carbides with Nb/TiC/TaC/VC and Co was prepared through spark plasma sintering (SPS) at a low sintering temperature of 1300°C, and their microstructures and mechanical properties were investigated. The nonstoichiometric multicomponent carbide Nb/TiC/TaC/VC with a rock‐salt structure (Fmtrue3¯m$Fm\bar{3}m$) has a high atomic solution capacity. In the sintering process, partial WC and Co may dissolve in Nb/TiC/TaC/VC. With a high concentration of carbon vacancies, Nb/TiC/TaC/VC plays a beneficial role as a mass transfer intermediary. Good mass transfer facilitates the formation of a more accommodating and stable bonding between WC, Nb/TiC/TaC/VC, and Co, thereby preserving the hardness of the sintered bulks and preventing the initiation and propagation of cracks. When 6 wt.% Nb/TiC/TaC/VC and 4 wt.% Co are added to WC, the sintered bulk with fine grains exhibits superior hardness (23.27 ± .63 GPa) and toughness (10.45 ± .56 MPa·m1/2).