Based on the single mode 1D HTC model, we have theoretically studied the motional narrowing effect using both the analytic linewidth expressions (Eqs. and the numerical exact simulations via HEOM. In the polaron decoupling regime, we have demonstrated that the polariton linewidth scales as 1/N under the fast limit, while scales as 1/ √ N under the slow limit, consistent with the recent work based on Kubo-Anderson theory . The scaling relations are verified by numerical exact simulations. Furthermore, both the UP and the LP exhibit a subaverage behavior, and UP is usually broader than LP mainly due to inter-branch scatterings, which are the two major characteristics of the motional narrowing effect. Our results agree well with the experimental trends . Moreover, the analytic linewidth expressions in Eqs. 34-35 agree well with the numerical exact results in all the parameter regimes we explored. In brief, our results reveal that motional narrowing effect can be adequately described using the single mode 1D HTC model, which is also closely related to the polaron decoupling effect .