Abstract: WC-based cemented carbides have been widely applied in cutting, molding, and mining industries since the 20th century due to their outstanding hardness and wear resistance. However, issues such as insufficient toughness, high fabrication cost, and limited service life—particularly under extreme environments—still restrict their broader applications. In this study, spark plasma sintering (SPS) combined with the in-situ carbothermal reduction of WO₃ was employed to introduce multi-walled carbon nanotubes (MWCNTs) as both a reinforcing phase and a grain growth inhibitor. The MWCNTs released active carbon atoms during sintering to regulate the carbon potential and stabilize the WC phase. Meanwhile, those distributed along grain boundaries formed barrier layers and interfacial structures, which collaboratively suppressed grain coarsening and abnormal growth, thereby enhancing the densification and overall performance of the material. By systematically adjusting MWCNT content, sintering temperature, holding time, and applied pressure, the evolution of phase composition, microstructure, mechanical properties, and magnetic behavior was thoroughly investigated. Results indicate that the incorporation of MWCNTs effectively suppresses grain coarsening, promotes liquid phase formation, and delays grain boundary migration, thereby facilitating gas release and structural rearrangement during sintering. Under optimized conditions (0.4 wt.% MWCNTs, 1400 ℃, 10 min holding time, and 40 MPa pressure), the synthesized WC-based composite exhibited fine and uniform grains, low porosity, and superior mechanical performance, achieving a hardness of 2053.2 HV, a fracture toughness of 13.5 MPa·m^1/2, a density of 14.83 g/cm³, and an average grain size of 175 nm. This work not only highlights the synergistic effect of MWCNTs and SPS in tailoring WC microstructures but also provides theoretical and technological reference for designing next-generation high-performance cemented carbides.