The silane coupling agent 3-aminopropyltriethoxysilane (KH-550) was used to modify the surface of two kinds of high-performance polyimide (PI) fibres (i.e., PI-1 and PI-2). The surface chemical composition, morphologies and roughness of modified PI fibres were characterised by XPS, SEM and AFM. Results showed that the elemental ratio between O and C, the surface oxygen concentration and the surface roughness increased with KH-550 concentration. However, the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) values of PI fibre/epoxy resin composites increased and then decreased with KH-550 concentration. When KH-550 concentration was increased to 4 wt%, the IFSS and ILSS values reached their maximum point. At this point, the IFSS values were found increase by 17.3% and 8.3% for PI-1 and PI-2, respectively, compared with their pristine state. Meanwhile, the ILSS values of PI-1- and PI-2-reinforced composites were found increase by 22.4% and 24.1%, respectively.