The processing properties， rheological properties and heat resistance of a high-temperature resistant modified bismaleimide resin （805） were studied. On this basis， TG800/805 domestic high-strength medium modulus carbon fiber composite was prepared， and the mechanical properties and fracture microstructure at high temperature and room temperature were characterized. The results show that the lowest viscosity of 805 is 1.6 Pa·s， which has excellent processing properties. TG800/805 composite has excellent mechanical properties. After heat treatment at 280 ℃， the mechanical properties retention rates of flexural strength and inter-laminar shear strength at 280 ℃ are 68% and 52%， and the glass transition temperature（T_g） of 805 is 356 ℃，indicating that TG800/805 can be used at 280 ℃.