High energy density solid‐state lithium batteries require good ionic conductive solid electrolytes (SE) and stable matching with high‐voltage electrode materials. Here, a highly homogeneous poly(1,3‐dioxolane) composite solid electrolyte (CSE) membrane that can satisfy the above‐mentioned requirements by in situ catalytic polymerization effect of yttria stabilized zirconia (YSZ) nanoparticles on the polymerization of 1,3‐dioxolane (DOL), is reported. The well‐dispersed YSZ nanoparticle catalyst leads to the polymerization conversion of DOL monomers up to 98.5%, which enlarges its electrochemical window exceeding 4.9 V. YSZ also significantly improves the room temperature ionic conductivity (2.75 × 10 −4 S cm −1 ) and enhances the cycle life of lithium metal anode. Based on this CSE, the Li(Ni 0.6 Co 0.2 Mn 0.2 )O 2 (NCM622)‐based solid‐state lithium battery shows a long cycle life over 800 cycles. This investigation encourages polymer SE toward practical high energy solid‐state batteries.