The invention relates to an SERS (surface enhanced Raman scattering) active droplet and a preparation method and molecular detection method thereof, in particular to an SERS active droplet based on candle ash nanoparticle structure, a preparation method of the SERS active droplet, and a molecular detection method using the SERS active droplet, and belongs to the technical field of SERS detection. The SERS active droplet according to the technical scheme provided herein comprises a droplet body and a plurality of SERS active metal composite nanoparticles enriched thereby, the SERS active metal composite nanoparticles are positioned in the droplet body or adhered to the surface of the droplet body; each of the SERS active metal composite nanoparticles includes a metal nanoparticle body. The ability to absorb and enrich gold nanoparticles can be achieved herein, the strict requirement of existing SERS molecular detection for focusing can be avoided, transferring is facilitated, test convenience is improved, the preparation process is simple, and the SERS droplet is highly controllable, widely applicable and good in safety and reliability.