Abstract: In order to research the method of toughening bismaleimide and its effects on properties, nano-SiO₂ was modified by supercritical ethanol (SCE-SiO₂), and its surface activity of SCE-SiO₂ were improved, 4,4'-diamino diphenyl methane bismaleimide (MBMI), 3,3'- diallyl bisphenol A (BBA) and bisphenol-A diallyl ether (BBE) were used to synthesize MBAE(MBMI-BBA-BBE) composite matrix, SCE-SiO₂/PES-MBAE multi-phase composite was prepared from adding SCE-SiO₂ and polyether sulfone (PES) into MBAE matrix by in-situ polymerization method and sol-gel method. SEM was used to observe the fracture morphology of SCE-SiO₂/PES-MBAE composites. The analysis result of FTIR shows that the absorption peak of Si—OH at 3 434 cm^-1 disappeares and the absorption peaks of —OH between ethanol molecules (3 310 cm^-1), —CH₃ and —CH₂— (2 984 cm^-1)appeares, this indicates the nano particle is coated with ethanol molecule in some form and surface properties improve. PES exists in the matrix as dispersed phase in the form of "honeycomb", the fracture modes transform from brittle fracture to ductile fracture. SCE-SiO₂ and PES have the toughing effect to the material, the toughing effect of PES are extremely obvious, and there are synergistic between them. When the content of SCE-SiO₂ is 2wt%, PES content is 4wt%, the impact strength and the bending strength of multi-phase composites are 15.02 kJ/m² and 130.47 MPa, enhance 57.3% and 35.8% than that of MBAE matrix, respectively. The dielectric property test shows that the dielectric constant and dielectric loss of SCE-SiO₂/PES-MBAE composites show a little increase while SCE-SiO₂ and PES exist, but the synergistic reaction between them can reduce the negative impacts of PES.