Abstract: To address the low room-temperature ionic conductivity and poor mechanical strength of poly(ethylene oxide) (PEO)-based solid-state electrolytes, this study proposes a synergistic modification strategy using boehmite (BM) and succinonitrile (SN). A series of boehmite samples with different grain sizes were synthesized via a hydrothermal method, and PEO-LiTFSI-SN-BM composite solid electrolytes were prepared by solution casting. The structure and properties of the electrolytes were systematically characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). The results show that the incorporation of SN and boehmite significantly reduces the crystallinity of PEO. Moreover, interactions between hydroxyl groups on the boehmite surface and the polymer chains as well as lithium salt anions establish favorable lithium-ion transport pathways. When the hydrothermal temperature of boehmite was 220℃ with an addition amount of 15wt.%, and the molar ratio of SN to ethylene oxide (EO) units was 1∶4, the obtained composite electrolyte (PL24S4B15-220) exhibited an ionic conductivity as high as 0.913×10⁻⁴ S·cm⁻¹ at 30℃, approximately two orders of magnitude higher than that of the pure PEO electrolyte (1.16×10⁻⁶ S·cm⁻¹). Simultaneously, its mechanical strength and electrochemical stability were significantly enhanced. Furthermore, the assembled LiFePO₄|PL24S4B15-220|Li battery demonstrated good cycling stability. This study provides a new approach for developing high-performance, low-cost polymer-based composite solid electrolytes.