Abstract: The ASTM 3059-18 standard incorporated high temperature residual mechanical properties into the flame retardant index of resin matrix composites, which broke through the traditional chemical flame retardant concept of composites, and marked that the concept of structural flame-retardant has been valued by the designer. In this work, the self-made polysiloxane modified epoxy resin (EP-Si) was blended with phenolic resin (PF), supplemented with inorganic powder and glass fiber reinforcement. The effects of polysiloxane modified epoxy resin and inorganic powder on the high temperature residual strength of glass fiber/phenolic composites were studied by means of mechanical properties, thermogravimetric analysis (TGA), cone calorimeter (CCT) and scanning electron microscope (SEM). The experimental results show that when the amount of EP-Si is 40wt%, the flexural strength and high temperature residual flexural strength of the composite is 384.4 MPa and 53.3 MPa, respectively, which is 78.7% and 85.1% higher than PF composite. With appropriate proportion of inorganic powder, the maximum residual flexural strength can reach 85.1 MPa, which is 195.5% higher than PF composite. After heat-treated, PF composite containing silicon expands along thickness, while PF composite shrinks along thickness. The pyrolysis residual rate of the PF composite containing silicon is higher and oxidative degradation of the surface layer is faster, but the content of CO generated in the inner layer is lower than that of PF composite. The inorganic pyrolysis product of resin matrix containing silicon protects inner layer resin and fibers. The distribution of the in-situ pyrolysis inorganic product is more uniform, the good compatibility with inorganic powder and possible co-sintering effect further isolates the oxygen intrusion, improves structural integrity and high temperature residual strength.