Abstract: In this study, the shear strength of 500-day long-age ultra-high toughness cementitious composites (UHTCC), reactive powder concrete (RPC), and the interface of UHTCC/RPC was tested by double-shear test. The failure process was observed by the digital image correlation technique. The results show that the UHTCC, RPC, and the interface of UHTCC/RPC exhibit good shear toughness and no brittle failure during loading. In addition, the improved casting process and increased roughness of the interface can increase the interfacial shear strength of UHTCC/RPC. Comparison of the existing empirical formulas for calculating the interfacial shear strength with the test results revealed that the existing empirical formulas could not predict the interfacial shear strength of UHTCC/RPC accurately. A finite element analysis model of UHTCC/RPC interface shear test was established and the interfacial behavior was modelled by COHESIVE elements. The simulation results were in good agreement with the test results.