Abstract: In order to improve the toughness of resin transfer molding(RTM) composites, combining the “ex-situ” composite toughening technology and RTM processing, a novel thermoplastic phosphorus-containing polyaryletherketone(PAEK-P) was used to toughen the carbon fiber/bismaleimide resin(CF/BMI) composites. The rheological properties, phase separation behavior of the PAEK-P-BMI resin and the effect of PAEK-P upon the toughness of the CF/BMI composites were investigated. The results show that the PAEK-P resin has high heat resistance at the glass transition temperature of 268.8℃ for its rigid structure. The gel time and viscosity increase inflection point time of the PAEK-P-BMI composite resin are little affected by the PAEK-P content because of its poor solubility in BMI resin. The PAEK-P-BMI composite resin shows no phase separation at 110℃/300 min. However, the phase separation structure is formed in the late high temperature curing process, and the phase separation morphology is maintained in the cured CF/PAEK-P-BMI composites. Compared with CF/BMI composites, the CF/PAEK-P-BMI composites have a 69% reduction in damage area after impact, a 16.6% increase in compressive strength after impact, and a 34.4% reduction in impact pit depth.