Herein, the study investigates the microstructure of recycled aggregates concrete (RAC) prepared by nano-silica strengthened recycled aggregates (RAs). Five concentrations (0 %,1.0 %,1.5 %,2%, 2.5 %) of nano-silica solutions in two different conditions (normal pressure and vacuum pressuring of 1ba) are selected to soak RAs. After the soaking process, the chemical composition, the porosity and pore structure, and micrographs of nano-silica strengthened RAs and RA-based concrete are investigated. Results show that the 2 % concentration alone with the vacuum pressing is optimal. It reduces CH content by more than 40 %, decreases porosity, narrows the average pore size, and increases the volume fraction of pores that are less harmful (less than 50 nm). These results, verified by ESEM, are attributed to the conversion mechanism of secondary hydration and the filling effect of nanoparticles. The research at a microscopic level explains the strengthening effect of nano-silica upon recycled aggregate concrete and lays a theoretical foundation for upcycling construction and demolition wastes.