Lithium–sulfur (Li–S) batteries hold significant promise as rechargeable energy storage systems due to their exceptionally high theoretical specific capacity and energy density. However, the widespread adoption of Li–S batteries has been impeded by challenges such as the diffusion of long-chain polysulfides and the formation of lithium dendrites when organic liquid electrolytes. To address these problems, a composite polymer electrolyte reinforced with Li6.28La3Al0.24Zr2O12 nanofiber (LLAZO NF) was developed. This electrolyte, featuring a garnet nanofiber filler within a PEO-based polymer system, exhibited superior ionic conductivity. The well-interconnected organic–inorganic network facilitated rapid and uninterrupted pathways for lithium-ion conduction, achieving a high Li-ion transference number. The incorporation of LLAZO NFs not only enhanced the electrochemical stability and mechanical properties of the composite polymer electrolyte, effectively mitigating lithium dendrite formation, but also contributed to the suppression of polysulfide diffusion during cycling. As a result, the all-solid-state Li–S battery utilizing this garnet-type composite polymer electrolyte demonstrated robust cycling stability and excellent rate performance at room temperature.