Extensive research has been conducted on plant-derived polysaccharides, yet three interrelated challenges remain insufficiently explored: extraction methods are often selected without systematic validation; the structure–activity relationship (SAR) remains fragmented; and modification strategies are rarely designed on the basis of SAR knowledge. This article systematically summarizes three key aspects: extraction, SAR, and structural modification. We critically compare various extraction techniques and clarify that method selection requires consideration of source characteristics and application orientation. We also demonstrate that extraction conditions may affect structural integrity and, consequently, influence biological activity. Subsequently, we analyze SAR by examining molecular weight, monosaccharide composition, glycosidic linkages, uronic acid content, and chain conformation to identify structural determinants of biological functions. Finally, we review chemical modification strategies and illustrate how their efficacy depends on structural features initially determined by extraction. By integrating evidence from these three stages, this review offers a practical framework to support rational, evidence-based development of plant-derived polysaccharides for applications in medicine, food, and biomaterials.