The cordierite–belite core-shell lightweight aggregate (CSLWA) effectively improves the performance of the interfacial transition zone (ITZ) in lightweight aggregate concrete; however, the underlying fracture behavior still needs to be revealed. This study characterized the reinforced mechanical properties of ITZ by CSLWA through nanoindentation and interfacial fracture tests and the reinforcement mechanism was further explained using finite element analysis. The results showed that the continuous hydration of the belite shell of CSLWA enhanced the yield stress and improved the failure strain during fracture. Energy dissipation was observed during the fracture of CSLWA concrete, improving the brittleness of the lightweight aggregate concrete. Finite element analysis showed that the core strength of CSLWA changed the energy dissipation model and affected the toughness of CSLWA concrete, and a suitable composition of 6:4 was proposed for the “cordierite–anorthite” core constitution in CSLWA depending on the strength order between the cement matrix and core of the CSLWA.