In this work, a facile and interesting fabrication of the Forster resonance energy transfer (FRET) system is presented for the first time by introducing an independent donor and acceptor into a confined space, which is different from the typical "donor-linker-acceptor" FRET system. Human serum albumin (HSA), as a proof-of-concept, is selected to load a pair of spectra-matchable but independent fluorescent dyes (ES1 and 111) in its different binding sites. Furthermore, this space-confined FRET system could act as a ratiometric sensor for the quantitative recognition of Ag+ in the water samples with good selectivity and high sensitivity.