Solid polymer electrolytes (SPEs) hold a great promise in the application of solid‐state lithium batteries, but suffer from poor mechanical properties and uncontrolled electrode/electrolyte interfacial reaction, which restrict their overall electrochemical performance. Herein, the design of 2D fluorinated graphene‐reinforced PVDF‐HFP‐LiTFSI (FPH‐Li) polymer electrolytes to address these challenges is reported. Uniformly dispersed fluorinated graphene induces a unique grain refinement effect, which effectively improves the mechanical properties without excessively increasing the thickness of the polymer electrolyte. Significant reduction in polymer grain size enhances interfacial lithium ion (Li‐ion) transport and homogenizes Li‐ion flux, thereby improving Li‐ion conductivity and promoting uniform Li plating/stripping. Furthermore, extensive characterizations show that fluorinated graphene is involved in the construction of a stable artificial interface, which effectively prevents the side reactions between the lithium metal anode and solvated molecules. As a result, the use of thin FPH‐Li polymer electrolytes (thickness of ≈45 µm) enables long‐term Li plating/stripping with a small overpotential in Li/Li symmetrical cells and stable cycling of Li/LiNi0.6Co0.2Mn0.2O2 full cells with a high average Coulombic efficiency of 99.5% at 1.0 C. This work verifies the effectiveness of 2D materials in improving the comprehensive properties of polymer electrolytes and promotes the applications of SPEs in high‐performance solid‐state lithium batteries. 2D fluorinated graphene is found to induce a unique grain refinement effect, which can improve the mechanical properties, increase the Li‐ion conductivity, and regulate the interfacial stability of the poly(vinylidene fluoride‐co‐hexafluoropropylene)‐based polymer electrolytes. The use of fluorinated graphene‐reinforced polymer electrolytes enables stable cycling of both symmetric cells and full cells at room temperature.