Heterocyclic aramid fibers, a typical kind of high-performance fibers, have been widely used in aerospace and protection fields because of their excellent mechanical properties. However, the application of heterocyclic aramid fibers as a reinforcement is hindered by the weak interfacial combination with matrix materials, especially epoxy. Traditional strategies enhancing the interfacial shear strength (IFSS) usually decrease the tensile strength. Therefore, simultaneous enhancement of both mechanical properties remains a challenge. Herein, we report a novel heterocyclic aramid fiber with high interfacial shear strength (49.3 MPa) and tensile strength (6.27 GPa), in which 4,4′-diamino-2′-chlorobenzanilide (DABA-Cl) and a small amount of graphene oxide (GO) are introduced through in-situ polymerization. Hydrogen bonds and π–π interaction between GO and polymer chains trigger the enhancement in crystallinity, orientation, and lateral interaction of the fibers, thus improving the tensile strength and interfacial shear strength of the fibers. Moreover, the interfacial interaction between fiber and epoxy is enhanced due to the improvement of the surface polarity of the fibers caused by DABA-Cl. Therefore, a method to improve both tensile strength and interfacial shear strength of heterocyclic aramid fibers was found by introducing GO and DABA-Cl, which may provide guidance for the design and preparation of other high-performance fibers.