Dynamic structural changes of bio-macromolecules undergoing biochemical reactions can be studied using the single-pair fluorescence resonance energy transfer (sp-FRET) experiments, which can be used as a nanoscale ruler. The ability to use the optical microcavity to amplify the sp-FRET fluorescence signal has been investigated. When the single FRET-coupled pair and the attached bio-macromolecule are placed in an optical cavity with the emission mode of the acceptor dye in resonance with the cavity mode, the results of a fully quantum-mechanical treatment show that, the photon emission intensity from acceptor amplified by the cavity coupling field mode would reach a significant value, leading to a much brighter photon signal. It is concluded that the microcavity can serve as a sharper structural-sensitivity observable tool to the distance of single pair. The work establishes experimental methods and useful guidelines for the single-molecule studies of biomacromolecules in real time at molecule scale.