Abstract: Solid-state lithium metal batteries (SSLMBs) have emerged as the core direction for next-generation energy storage due to their high energy density and intrinsic safety, with solid-state electrolytes (SSEs) serving as the key enabler for performance breakthroughs. Polymer-in-salt (PIS) electrolytes overcome the bottleneck of low room-temperature ionic conductivity in traditional polymer electrolytes by adjusting the polymer-to-lithium salt ratio (mass fraction of lithium salt ≥50%) and constructing fast lithium-ion transport channels. Polyvinylidene fluoride (PVDF) has become an ideal matrix material for PIS electrolytes owing to its high dielectric constant, excellent lithium salt solubility, mechanical strength, and electrochemical stability. However, single-component systems suffer from inherent drawbacks such as phase separation, imbalanced mechanical strength, and insufficient interfacial compatibility. This review systematically summarizes the performance advantages and ion transport mechanisms of PVDF-based PIS electrolytes, with a focus on elucidating the working principles and research progress of four core modification strategies: lithium salt selection and ratio optimization, multifunctional inorganic filler compositing, interface construction and optimization, and polymer blending. Finally, it outlines the core challenges facing PVDF-based PIS electrolytes in terms of performance adaptability and large-scale fabrication, prospects future directions related to mechanism refinement and structural innovation, and provides references for the development of high-performance PVDF-based PIS electrolytes and the practical application of SSLMBs.