Conventional precursor infiltration and pyrolysis (PIP) methods for fabricating C/SiOC composites often resulted in high material density due to excessive ceramic particle accumulation within fiber pores. To address this issue, we proposed a novel precursor infiltration-vacuum filtration-pyrolysis (PIVFP) process, which selectively removed redundant ceramic precursors from fiber voids while retaining critical ceramics at fiber junctions. By introducing vacuum filtration after infiltration, this method enabled precise control over ceramic distribution, reducing overall density while maintaining mechanical performance. The optimized PIVFP process produced composites with a density as low as 0.31 g/cm3 (a 39% reduction compared to the traditional PIP process, which yielded a density of 0.51 g/cm3) and a ceramic volume content of only 3% (versus 12.5% in PIP-processed composites). Although the absolute compressive strength slightly decreased by 11%, the specific strength increased by 47% due to the significant weight reduction. Microstructural analysis confirmed that ceramics were preferentially retained at fiber interlocks to ensure efficient load transfer, while minimizing ceramic filling in pores to reduce thermal conductivity. This work demonstrates a scalable production strategy for lightweight ceramic matrix composites, achieving a balance between mechanical and thermophysical properties.