This study aims to determine the optimal extraction process for polyphenols from Ganoderma applanatum, identify its polyphenolic components, and explore its in vitro antioxidant activities. Initially, ultrasonic-assisted extraction was employed, with process parameters optimized through single-factor experiments followed by response surface methodology. Subsequently, UHPLC-ESI-MS was used for compound identification. Furthermore, the contents of total polyphenols and total flavonoids in seven strains of G. applanatum were determined, while the scavenging activities of ABTS and DPPH free radicals, ferric reducing/antioxidant power (FRAP), and lipid peroxide inhibition activity were measured. Finally, correlation analysis was performed on these indices. The results demonstrated that the optimal extraction conditions were a material-liquid ratio of 1∶50 (g/mL), ethanol concentration of 68%, ultrasonic time of 45 min, and ultrasonic temperature of 50 ℃, yielding a polyphenols content of 8.77 ± 0.27 mg/g. Notably, twenty-nine polyphenolic compounds were identified by UHPLC-ESI-MS. Regarding antioxidant capacity, sample 7 exhibited the strongest activity among the seven strains: IC₅₀ values for DPPH and ABTS radical scavenging were 39.36 µg/mL and 101.28 µg/mL, respectively; FRAP value reached 0.54 mmol Trolox/mg; and lipid peroxidation inhibition was 47.51%At 200 µg/mL. However, all values remained significantly lower (P < 0.05) than the positive control V_C. Correlation analysis revealed strong associations: total polyphenols strongly correlated with total flavonoids (r = 0.842) and further showed positive correlations with with DPPH, ABTS radical scavenging rate, FRAP and lipid peroxidation inhibition (r = 0.932-0.966). In contrast, total flavonoids only strongly correlated with FRAP (r = 0.942). Collectively, this study provides a theoretical and experimental foundation for further research and utilization of G.applanatum.