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Phase decomposition can effectively enhance the mechanical properties of carbide ceramics and can overcome the difficulty of enhancing the mechanical properties of single-phase multicomponent carbide ceramics. In this work, a series of nonstoichiometric (TiZrVNb)Cx ceramics were prepared by spark plasma sintering (SPS) at different temperatures. The effects of the carbon content on the phase composition, microstructure evolution, and mechanical properties were investigated in detail. Phase decomposition occurred with decreasing carbon content. Two different solid solutions of (Ti,V)-rich and Zr-rich phases formed from the decomposition of equimolar single-phase solid solutions, namely, the Zr-poor phase and Zr-rich phase, respectively. The distribution of Nb element is relatively uniform. The semicoherent interfaces between the Zr-poor phase and the Zr-rich phase can harden and strengthen effectively under the synergistic effect of grain refinement. Ceramics with phase decomposition structures have apparent advantages compared to single-phase high-entropy carbides. This work provides an important train of thought for the microstructure tailoring and properties optimization of multi-component carbide ceramics.