The Ex-situ elasticizer polyethersulphone(PES) was adhered homogeneously on carbon fabric according to Ex-situ toughened technology. Carbon fiber reinforced BMI composites U3160/6421 was prepared by RTM process. The effect of PES on the properties of U3160/6421 composites was investigated through low-velocity, DMA, SEM and mechanical properties tests. The results showed that composites prepared by ES-Fabrics adhering PES had a good toughening effect. The CAI value of U3160/6421 composites toughened by PES increased by 85%. Because of the added PES, the phase version of PES and BMI6421 resin appeared in the interplay of composites. This structure improved the damage resistance and damage tolerance of composites, and Ex-situ interlaminar-toughening had a little impact on mechanical properties.