In today’s global pursuit of green and low-carbon development, the demand for sustainable and environmentally friendly energy-storage technologies has become increasingly urgent. Against this backdrop, a nature-derived material—cellulose-based materials—are emerging as a promising new star in the field of flexible supercapacitors. Although flexible supercapacitors hold great promise thanks to their flexibility and lightweight, thin characteristics, they still face significant bottlenecks, including insufficient energy density, poor environmental adaptability, and challenges in ensuring safety, all of which limit their large-scale application. Currently, the academic community lacks a comprehensive review that systematically elucidates the intrinsic link between the “chemical properties” of cellulose materials and their “performance outcomes” in devices. This is precisely the motivation behind our current study. To fill this gap, we have conducted a thorough synthesis of cellulose and its derivatives. We not only summarize their physicochemical properties and assembly methods but also delve deeper into the pathways through which they can be functionalized via chemical techniques such as oxidation, esterification, and graft polymerization, revealing how cellulose—a seemingly ordinary biomass material—can be transformed into high-performance energy-storage components. More importantly, this review serves as a “roadmap for applications”... Map The paper provides a detailed overview of how various cellulose materials can be precisely integrated into key components of flexible supercapacitors, including electrodes, electrolytes, separators, and binders, and elucidates the intrinsic “structure-function” relationship underlying these applications. Finally, given that large-scale application of cellulose-based flexible supercapacitors is still in its early stages, we proactively propose a holistic design framework that integrates “chemistry, performance, and sustainability.” This framework not only addresses the current limitations of existing devices but also lays out a clear roadmap for the development of next-generation flexible supercapacitors that are truly green and high-performance. We believe that by clarifying the logical chain—from “chemical structure” determining “device performance” to ultimately achieving “environmental sustainability”—we can significantly advance flexible electronics toward greener and more practical applications.