Due to the favorable interfacial stability with electrodes, excellent processability, and reasonable material cost, organic–inorganic composite solid-state electrolytes have attracted broad interests in the field of solid-state batteries. In this study, we have developed a solid-state composite electrolyte with polyurethane (PU) as polymer matrix and TiO_2−x as nanofiller (denoted as PUL-TiO_2−x). The block copolymer PU features alternating soft and hard segments, which offers distinct advantages due to its unique structural arrangement. The soft segment of the block copolymer facilitates the dissociation of lithium salt, enabling the conduction of Li⁺, while the rich hydrogen bond network formed by the hard segment ensures the mechanical strength of the electrolyte. The profusion of Lewis acid sites on the TiO_2−x surface facilitates interactions with ether oxygen groups and bistrifluoromethanesulfonimide (TFSI⁻) anions, thereby enhancing ionic conductivity (σ) and expanding the electrochemical stability window of the electrolyte. Notably, the PUL-TiO_2−x electrolyte exhibits an impressive σ of 2.19 × 10⁻⁴ S·cm⁻¹ at 40 °C, a Li⁺ transference number of 0.47, and an electrochemical stability window of 4.98 V. The resulting LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)||Li battery demonstrates a specific capacity of 171 mAh·g⁻¹ and exhibits excellent cycling stability, maintaining its performance over 270 cycles at 40 °C. These findings underscore the immense potential of the PUL-TiO_2−x in advancing the development of high-performance all-solid-state lithium batteries.