Abstract: Recycled concrete aggregate (RCA) is difficult to be applied on a large scale in engineering structures due to its inherent defects such as the adhesion of old mortar on its surface, high porosity and large water absorption. To address this issue, this paper systematically reviews the research status of RCA strengthening technologies at home and abroad, and compares and analyzes the two mainstream paths of "enhancing old mortar" and "stripping old mortar", as well as the mechanism of action, performance improvement and engineering applicability of the optimized mixing process and composite strengthening schemes. The research finds that some composite strengthening technologies have achieved significant quality improvement effects through synergy, such as the "internal and external" dual strengthening pretreatment method: the internal carbonization reaction generates products to fill and repair the micro-pore defects of the recycled aggregate itself; the external coating of modified slurry not only provides a stable structural support for the aggregate but also forms a dense and high-strength interface transition zone with the fresh mortar, thereby significantly improving the mechanical properties and microstructure compactness of the recycled aggregate and recycled aggregate concrete. Although the existing strengthening methods can improve the performance of recycled aggregates to a certain extent, they generally face practical challenges such as complex processing procedures, high economic costs, potential environmental impacts or unclear long-term performance. In the future, it is necessary to break through the above technical bottlenecks, develop low-cost and low-environmental-impact green composite strengthening processes, and establish long-term performance prediction models to break through the bottleneck of large-scale application and provide technical support for the resource recycling and green low-carbon transformation of the construction industry.