In this study, first-principles calculations were performed to study the stability, mechanical property, electronic structure and lattice dynamics of beta-Si-3(C-x,N1-x)(4) silicon carbonitride. The solubility of carbon in beta-Si-3(C-x,N1-x)(4) having a stable structure is shown to be about 15 at.%. Within the limit of solubility, an increase in carbon concentration in beta-Si-3(C-x,N1-x)(4) will lead to a decrease in the Young's modulus and density and an increase in the Poisson's ratio. The study of deformation behavior shows that the most likely slip system of {1100} [0001] is on prismatic plane rather than on basal plane. This feature of beta-Si-3(C-x,N1-x)(4) is similar to WC. In addition, the ductility and fracture toughness of beta-Si-3(C-x,N1-x)(4) can be optimized by controlling the carbon concentration. The improvement in ductility and fracture toughness can be attributed to the formation of metallic bonds by the incorporation of carbon atoms. The lattice dynamics study shows that the structural stability of beta-Si-3(C-x,N1-x)(4) is controlled by energy stability criteria under stress-free condition. In the stressed state, the structural stability of beta-Si-3(C-x,N1-x)(4) is controlled by the elastic stability criteria. Subsequently, the beta-Si-3(C-x,N1-x)(4) solid solution was prepared by self-propagating high-temperature synthesis (SHS), and the carbon-concentration-dependent mechanical properties were consistent with the first-principles calculations. The maximum fracture toughness of 10.4 MPa.m(0.5) was obtained in beta-Si-3(C-x,N1-x)(4) at carbon concentration of 5 wt%, which means that the solid solution toughening can be used as a supplement to crack bridging toughening and phase transition toughening for ceramic toughening. The results obtained in this study reveal that the beta-Si-3(C-x,N1-x)(4) solid solution is a promising candidate for high-speed ceramic bearings.