Abstract: Natural Hydraulic Lime (NHL) has exceptional application in restoring ancient buildings compared to Portland cement and conventional air-hardening lime, but the characteristic of slow early performance development limits its fields of application. This paper studied the physical and mechanical properties, hydration exothermic properties, phase composition and transformation, and microstructure evolution of slag powder/metakaolin composite NHL during the early hardening process. The influence of slag powder/metakaolin on the hardening process and performance development of NHL are systematically evaluated. Results show that slag powder can improve the fluidity of NHL-based mortar; slag powder/metakaolin generates calcium aluminate hydrate (C₃AH₆), calcium carboaluminate hydrate (C₄AĈH₁₁) and calcium silicate hydrate (C—S—H) through pozzolanic reaction to promote the setting and hardening of NHL-based materials and significantly improve their compressive and flexural strength. This study provides reference for promoting the application of pozzolanic material composite NHL in the restoration of ancient buildings.