X-ray computed tomography (XCT) technology was applied to systematically characterize the evolution of the microstructure of Si3N4f-Cf/BN composites throughout the precursor infiltration and pyrolysis (PIP) process. The research specifically focuses on the temporal evolution of both the matrix and porosity during various stages of the PIP process, providing a detailed analysis of how these two critical components interact and evolve over time. In particular, the study delves into the filling mechanism of the matrix as it relates to the existing pores within the composite structure. By examining this relationship, we gain insights into how the matrix material infiltrates and occupies the pores, which ultimately influences the overall properties of the composite. Furthermore, building upon the foundational understanding established through XCT analysis, the study explores the mechanisms underpinning the densification process of Si3N4f-Cf/BN composites during PIP. This exploration is achieved by integrating pore network modeling with permeability models and monitoring changes in absolute permeability throughout the process. Through this multifaceted approach, the research elucidates the complex interactions that dictate the densification behavior of the composites under study. The findings from this investigation not only enhance the theoretical understanding of the microstructural evolution in Si3N4f-Cf/BN composites but also provide valuable experimental references for the optimized fabrication and processing of related composite materials.