Polysaccharides are natural polymers widely found in medicinal plants. Structurally, polysaccharides are complex molecules composed of long chains of monosaccharide units linked by glycosidic bonds. Modern pharmacological research shows that the monosaccharide composition of polysaccharides is closely related to their bioactivity. This review systematically summarizes high-quality literature from recent years concerning the influence of monosaccharide composition on the biological activity of polysaccharides. The results are as follows: Glucans are vital for immune regulation, antioxidant effects and the regulation of gut microbiota. Similarly, galactans are important for antioxidant effects, immune regulation and the regulation of gut microbiota. Mannans play a critical role in immune regulation, anti-tumour effects and neuroprotection. Fructans play a critical role in regulating gut microbiota, immune regulation, and antioxidant effects. Pectin plays a critical role in immune regulation, antioxidant effects, and lowering blood sugar. Consequently, developing polysaccharides from medicinal plant resources based on their monosaccharide composition is expected to speed up the search for polysaccharides with high biological activity and provide a theoretical reference for polysaccharide research.