The development of solid-state electrolytes (SSEs) with high ionic conductivity, outstanding electrochemical window, and promising mechanical strength is a key factor in realizing the commercialization of high energy density solid-state lithium metal batteries (LMBs). Covalent organic frameworks (COFs) are a functional crystalline material with highly customizable molecular networks and one-dimensional channel structures, thus showing great potential applications in SSEs. Herein, we design flexible COF-poly(vinyl ethylene carbonate) (PVEC) (abbreviated as COF-PVEC) composite electrolyte films with excellent ionic conductivity and high mechanical strength, enabling dendrite-free and long-term running solid-state LMBs. Owing to the lithium-philic triazine and carbon–carbon double bonds groups in the COF skeleton, the obtained flexible COF-PVEC shows high ionic conductivity up to 1.11 × 10⁻⁴ S·cm⁻¹ at 40 °C, and enlarged electrochemical window up to 4.6 V (vs. Li⁺/Li) compared with pure PVEC electrolyte. At the same time, the lithium dendrites are efficiently inhibited after discharge–charging cycles, due to the improved Young’s modulus (150 MPa) and ordered channels of COF. Using the various features of COF-PVEC, we assembled a solid-state full battery with LiFePO₄ cathode, which showed superior rate capacity (151.8, 146.2, 139.2, 128.1, 113.7, and 100.8 mAh·g⁻¹ at 0.1, 0.2, 0.5, 1, 1.5, and 2 C, respectively) and excellent long-term cycling stability (over 400 cycles at 1 C). We believe that the COF-based composite electrolyte can become one of the most promising high-performance SSEs for solid-state LMBs.