This work systematically investigated the performance evolution and damage mechanism of boron-modified 2.5D SiC/Si-B-C composites after long-term corrosion for 50h、100 h、150h、200 h、250h和300 h at 1100°C under extreme conditions with high water-oxygen partial pressure (90 kPa H₂O–10 kPa O₂). By analyzing the density, apparent porosity, room-temperature flexural properties and microstructure of the materials before and after corrosion, the results show that in the short and medium-term corrosion stages (≤200 h), the material properties remain stable, with the flexural strength retention rate exceeding 97%, and the apparent porosity slightly decreases. Boron achieves dynamic self-healing by forming borosilicate glass phase, effectively delaying the invasion of the oxidizing medium and showing a positive protective effect. However, after long-term corrosion (300 h), the material properties significantly deteriorate, with the flexural strength retention rate sharply dropping to 40%, the density decreasing to 2.40 g/cm³, and the apparent porosity increasing to 5.2%. The fiber pull-out length at the fracture surface significantly shortens, presenting brittle fracture characteristics, indicating severe oxidation damage at the fiber/matrix interface and failure of the composite material's toughening mechanism. Microscopic analysis reveals that the continuous volatilization of boron leads to matrix loosening and causes the surface CVD SiC coating to transform from a "cauliflower-like" structure to a molten glass state, generating macroscopic cracks and triggering failure, accelerating the overall performance degradation of the material. This study reveals the possible performance degradation mechanism of boron-modified SiC/SiC composites in long-term extreme water-oxygen environments, providing a theoretical basis for their engineering applications.