The term motional narrowing is first put forward by Bloembergen, Purcell, and Pound in the 1940s, which is a counter-intuitive phenomenon that spectral lines become sharper and narrower the more frequently the nuclear spins are disturbed in nuclear magnetic resonance (NMR) spectra. Later, this concept is used to explain the lineshapes in all facets of spectroscopy, in all research fields and across all frequency bands . The motional narrowing effect cavity exciton-polaritons had recently attracted attention, due to the fact that polariton has a much smaller effective mass and shorter lifetime compared to exciton, thus exhibiting more significant quantum mechanical effects and causes spatial averaging of disorder potential. Specifically, the disorder can be suppressed under strong light-matter coupling, reducing the width of spectral lines (which correspond to some quantum transitions). The first experimental find of cavity exciton-polariton motional narrowing effect is achieved by Whittaker et al. , where a characteristic subaverage behavior of the polariton linewidths (which means narrower than the average of the cavity and exciton linewidths) are observed. Moreover, the upper polariton (UP) and the lower polariton (LP) exhibit different narrowing behavior, that the UP is always broader than the LP. These universal behaviors observed in experiments cannot be explained by simply averaging the exciton and photon linewidths, as would be valid if both were homogeneously broadened . Other experimental work was quickly and extensively followed up later .