The invention discloses an in-situ synthesis method of DNA silver nano-clusters Cells are subjected to DNA transfection by raising the level of glutathione (GSH) in cells, and therefore the cells are induced to synthesize the DNA silver nano-clusters in situ. The DNA silver nano-clusters synthesized in situ by the method have high biocompatibility, and the retention time of probes in the organisms is prolonged, so that the DNA silver nano-clusters can used for long-term monitoring over the activity of telomerase during the proliferation and differentiation processes of cells. In addition, by replacing sliver nano-clusers modified by oligonucleotide with different sequences as multifunctional fluorescent probes, the method can be used for detecting other functional molecules such as enzymes, glucose, protein, DNA and the like in a high-sensitivity mode and widely applied to the fields of bio-marking, disease monitoring and the like.