Noble metal nanomaterials have been studied by many researchers for their ultra-small size, excellent photophysical properties and good biocompatibility. Metal nanoclusters are a kind of nanoscale ultrafine particle, which have completely different properties from macroscopic metals. In the visible region, they do not usually show the characteristic surface plasmon resonance absorption but instead show fluorescence in the visible to near infrared region. In particular, the noble metallic (Au, Ag, Cu, etc) nanoclusters (NMNCs) have broad application prospects in the field of biomedicine as probes for fluorescence sensing. Their strong photoluminescence, living cell compatibility, and easy availability make up for the shortcomings of traditional fluorescent probes such as organic fluorescent dyes, fluorescent proteins, and fluorescent quantum dots. In this review, we summarize the synthetic method and application of metal nanoclusters as fluorescent probes in bio-sensing and imaging, especially in the early diagnosis of cancer cells.