Biomass structural materials can effectively address the issues of high energy consumption and environmental degradation brought by traditional engineering structural materials. However, natural structural materials often suffer from drawbacks such as low mechanical performance and flammability. Therefore, this study has developed an ultra-strong fire-resistant bamboo composite (UFBC). Natural bamboo (NB) was used as the raw material. After delignification treatment, bamboo fibers are grafted with epoxy groups through in-situ chemical bonding. Subsequently, polymer chains underwent in-situ chemical cross-linking within the bamboo fiber framework, combined with reinforcement from nano silica, resulting in strengthened cell walls. In addition, the softened and expanded cell walls can facilitate the deposition of phosphate and borate salt on the cell walls, forming an N-P-B flame-retardant system within the system. The tensile strength (463 MPa vs NB 112 MPa) and flexural strength (655 MPa vs NB 157 MPa) of UFBC increased fourfold, with a Limiting Oxygen Index (LOI) of 54.4%. Compared to similar bamboo-based composite materials, UFBC exhibits superior environmental friendliness and sustainability throughout its lifecycle, with all 18 environmental factors being optimized (up to a 92% reduction). This study provides an important reference for the application of high-performance biomass structural materials in construction and industry.