We propose a scheme of exploring the kinetic properties of the receptor-ligand binding process by means of the frequency stability measurement of a whispering-gallery-mode optical microcavity. In this scheme, the probe beam is directly locked to the microcavity and the time resolution of the sensing detection is determined by the feedback loop response time 10μs, which is much shorter than the current limit of about 20 ms. The receptor-ligand interactions are entirely mapped onto the probe light frequency and can be recovered through the analysis of its Allan deviation. As a result, one can investigate a chemical and biological reaction at the single-molecule level without the need for resolving the occurrence and duration of individual binding events. We numerically study this sensing scheme based on a practical optoplasmonic platform, where several gold nanorods are adsorbed onto the surface of a spherical microcavity that is located in the aqueous environment. The microsphere has a typical radius of 40μm and quality factor of 10^{6} (finesse of 2.6×10^{3}) at the wavelength of 785 nm. This powerful scheme may extend single-molecule sensing to the kinetic regime of the biochemical reactions.