DNA sequences containing a polycytosine (Poly-C) can be strongly adsorbed on the surface of graphene oxide (GO) to enhance the physical adsorption between GO and DNA. Hg²⁺ can specifically bind to single-stranded DNA sequence with thiophosphate cleavage site (PS), and fluorescence is restored according to the principle of fluorescence resonance energy transfer (FRET). The LOD of Poly-C PS 1 site sensor was 1.11 nM. The LOD of Poly-C PS 2 sites sensor was 0.84 nM. In order to improve the detection signal of Hg²⁺, the ultra-thin silica shell (SiO₂) was covered with metal nanomaterial Ag (Ag@SiO₂), which improved the stability and the sensitivity of biosensors. The LOD of the two sensors decreased to 0.36 nM and 0.15 nM, respectively. This sensor was also used for Hg²⁺ detection in dendrobium.This shows that the method is promising.