Abstract: An accurate analysis of the interlaminar stress state in composite materials is crucial for structural optimization. This paper focuses on thermoplastic resin-based composites, specifically examining the resin-rich region between layers, and presents a model for the transmission of interlaminar shear stress. Adopting an “overall first, then microelement” analysis approach, the model explores how external loads are transferred into internal forces within a laminated structure containing a resin-rich region. Experimental results demonstrate that, compared to the classical laminate theory model that neglects the resin-rich region, this model offers significantly improved accuracy. Using this model, the paper investigates the effect of single-ply thickness on the bending performance of laminated structures, focusing on failure load and the critical span at which bending/shear failure mechanisms shift. The findings indicate that reducing ply thickness helps mitigate bending normal stress transmission, lower interlaminar shear stress, and enhance failure load; on the other hand, at the same laminate thickness, it significantly increases the ratio of normal stress to interlaminar shear stress, making bending failure more likely. Thus, using thin prepreg for the fabrication of thermoplastic composites is recommended to enhance their bending performance. This research contributes to the advancement of composite material mechanics and offers valuable insights for improving the interlaminar shear strength of thermoplastic composites.