Grifola frondosa (hen of the woods or maitake) is a famous culinary-medicinal mushroom, and its exopolysaccharides (EPSs) have biological activities with or without supplementation with exogenous additives. In this study, a Rhizoma gastrodiae extract was added to a G. frondosa fermentation system. P-hydroxylbenzaldehyde (HBA), the main product of R. gastrodiae, had the highest utilization rate in the fermentation process (42%). In addition, the EPSs of G. frondosa after addition of R. gastrodiae extract (REPS), of HBA (HEPS), or of a standard solution according to the main component ratio of R. gastrodiae extract (CEPS) were obtained. We then determined the antioxidant and immunomodulatory activities of EPS, REPS, HEPS, and CEPS. Overall, REPS showed the highest antioxidant activities compared with EPS and HEPS (P < 0.05) but similar to that of CEPS (P > 0.05). The half-inhibitory concentration (ED50) values of REPS (< 4 mg/mL) were lower than those of EPS, HEPS, and CEPS. Moreover, REPS was better able to stimulate phagocytosis and nitric oxide production of RAW 264.7 macrophages than were the others, without a significant difference from CEPS (P > 0.05). An interesting and important finding is that a R. gastrodiae extract can increase antioxidant and immunomodulatory activities of EPS preparations from G. frondosa, and the standard solution of the main components of the R. gastrodiae extract may be better for simulating fermentation performed by G. frondosa and biological activities of its major products.