In this work, we adopt the 1D Holstein-Tavis-Cummings (HTC) model with single cavity mode to study the motional narrowing effect under a variety of different parameter regimes, based on a system-bath formalism of open quantum systems. We performed numerically exact quantum dynamics simulations using the hierarchical equations of motion (HEOM) for the linear absorption (LA) spectra of the single mode 1D HTC model. In particular, the polaron decoupling effect has been investigated and applied to explain the spectral linewidths narrowing phenomena . Our theoretical analyses reveal that in the polaron decoupling regime, the polariton linewidth scales as 1/ √ N under the slow limit which agrees with the Kubo-Anderson theory in Ref. 31, and scales as 1/N under the fast limit. Both the UP and the LP are subject to motional narrowing of the spectral response, and this effect for UP is less significant than LP due to inter-branch transitions (mainly to the dark states) that the UP branch exhibits. Moreover, we provide modified analytic expressions (Eqs. to describe the subaverage behavior of the polariton linewidths under general detuned cases. Our analytic expressions are in excellent agreement with the results obtained from numerical exact simulations. As a consequence, our results show that a single mode 1D HTC model will be adequate to exhibit a significant motional narrowing effect featured by the subaverage behavior of the polariton linewidths for both UP and LP, without inclusion of the full dispersion band, and to exhibit the experimentally observed asymmetry between the UP and LP branches.