Composite polymer electrolytes (CPEs) utilizing fillers as the promoting component bridge the gap between solid polymer electrolytes and inorganic solid electrolytes. The integration of fillers into the polymer matrices is demonstrated as a prevailing strategy to enhance Li-ion transport and assist in constructing Li -conducting electrode-electrolyte interface layer, which addresses the two key barriers of solid-state lithium batteries (SSLBs): low ionic conductivity of electrolyte and high interfacial impedance. Recent review articles have largely focused on the performance of a broad spectrum of CPEs and the general effects of fillers on SSLBs device. Recognizing this, in this review, after briefly presenting the categories of fillers (traditional and emerged) and the promoted ionic conducting mechanisms in CPEs, the progress in the interfacial structure design principle, with the emphasis on the crucial influence of filler size, concentration, and hybridization strategies on filler-polymer interface that is the most critical to Li-ion transport is assessed. The latest exciting advances on filler-enabled in situ generation of a Li -conductive layer at the electrode-electrolyte interface to greatly reduce the interfacial impedance are further elaborated. Finally, this review discusses the challenges to be addressed, outlines research directions, and provides a future vision for developing advanced CPEs for high-performing SSLBs.