Fabricating SiC matrix with controllable Si/C ratio for C/C-SiC composites via precursor infiltration and pyrolysis is difficult to realize due to the absence of suitable precursors. Here, SiC precursors with Si-rich (Si/C=1.23), near-stoichimetric (Si/C=1.01) and C-rich (Si/C=0.88) pyrolyzed ceramics at 1200 °C were successfully synthesized by the novel modification of polymethysilane. The structure and ceramization of synthesized SiC precursors were studied, and C/C-SiC composites with similar Si/C ratio in SiC matrix were also fabricated. The results revealed that the defects and Young's modulus of SiC matrix and the thermal residual stress on pyrolytic carbon coated carbon fiber were determined by the Si/C ratio. As a result, the Si/C ratio exerted significant effects on the crack deflection behavior and mechanical properties of the composites. The composites with near-stoichiometric SiC matrix showed excellent mechanical properties because of the effective crack deflection in composites and the moderate Young's modulus of their SiC matrix, the flexural strength, modulus and compressive strength were 423±27 MPa, 33.41±4.52 GPa and 393±8 MPa, respectively. Meanwhile, after a heat treatment at 1600 °C, the role of carbon fiber toughening for all composites was improved and accompanied by a decrease in mechanical properties, which was attributed to the increased defects in SiC matrix and the damaged interfaces by the thermal residual stress.