Thin solid‐state electrolytes with nonflammability, high ionic conductivity, low interfacial resistance, and good processability are urgently required for next‐generation safe, high energy density lithium metal batteries. Here, a 3D Li6.75La3Zr1.75Ta0.25O12 (LLZTO) self‐supporting framework interconnected by polytetrafluoroethylene (PTFE) binder is prepared through a simple grinding method without any solvent. Subsequently, a garnet‐based composite electrolyte is achieved through filling the flexible 3D LLZTO framework with a succinonitrile solid electrolyte. Due to the high content of garnet ceramic (80.4 wt%) and high heat‐resistance of the PTFE binder, such a composite electrolyte film with nonflammability and high processability exhibits a wide electrochemical window of 4.8 V versus Li/Li+ and high ionic transference number of 0.53. The continuous Li+ transfer channels between interconnected LLZTO particles and succinonitrile, and the soft electrolyte/electrode interface jointly contribute to a high ambient‐temperature ionic conductivity of 1.2 × 10−4 S cm−1 and excellent long‐term stability of the Li symmetric battery (stable at a current density of 0.1 mA cm−2 for over 500 h). Furthermore, as‐prepared LiFePO4|Li and LiNi0.5Mn0.3Co0.2O2|Li batteries based on the thin composite electrolyte exhibit high discharge specific capacities of 153 and 158 mAh g−1 respectively, and desirable cyclic stabilities at room temperature. Thin, flexible, and nonflammable composite solid electrolytes with plastic crystals in a 3D garnet‐based framework are prepared by a facile, solvent‐free method, and these unique composite solid electrolytes with high ionic conductivity and low interfacial resistance endow LiFePO4|Li and LiNi0.5Mln0.3Co0.2O2|Li cells with high discharge specific capacities, and desirable cyclic stabilities at room temperature.