Abstract: Stropharia rugosoannulata is rich in protein, polyphenols, and polysaccharides, making it a promising raw material for the development of functional foods; however, the effects of different harvest maturity stages on their quality and functional activity remain unclear. To identify high-quality ingredients, this study compared the nutritional composition, powder properties, amino acid profile, flavor activity values, content of bioactive compounds, antioxidant capacity, and α-amylase inhibitory activity of close-cap S.rugosoannulata powder (CSR) and open-cap S.rugosoannulata powder (OSR), and identified the components using LC-MS. The results showed that the protein content (27.06%), total amino acids (20.63 g/100 g), and total free amino acids (34.28 mg·g-1) in CSR were all significantly higher than those in OSR (20.54%, 15.45 g/100 g, and 16.35 mg·g-1); CSR also exhibited a higher L* of 88.36 and a superior water-holding capacity of 5.47 g·g-1. In terms of flavor, the TAV value of glutamic acid in CSR (15.80) was significantly higher than that of OSR (8.57), indicating a more pronounced umami taste. Regarding bioactive compounds, CSR had higher levels of total polyphenols (9.35 g/100 g) and crude polysaccharides (10.12 g/100 g), and its DPPH and ABTS+ radical scavenging rates (87.78%, 86.76%) and α-amylase inhibition rate (64.10%) were all significantly superior to those of OSR (77.75%, 76.73%, 34.90%); LC-MS analysis revealed that CSR contains a richer variety of polyphenols, including 6-gingerol and epicatechin. In summary, the nutritional composition, flavor compounds, antioxidant activity, and enzyme-inhibitory activity of CSR were superior to OSR, making it a high-quality raw material for the development of functional foods. This study provides a theoretical basis for the scientific harvesting and high-value utilization of S.rugosoannulata.