Medium additives have been shown to affect the synthesis of active products in fungi. This study investigated the effects of corn stalk, poplar sawdust, Tween-80, and oleic acid on mycelial biomass and physicochemical properties, as well as the bioactivity of polysaccharides, including exopolysaccharides (EPS) and intracellular polysaccharides (IPS), in the submerged culture of Bjerkandera fumosa. Results showed that the addition of corn stalk or poplar sawdust increased the production of EPS but decreased the production of IPS; Tween-80 had less effect on the production of EPS and IPS; and oleic acid stimulated polysaccharide production significantly. Polysaccharide property analysis showed that the addition of corn stalk or poplar sawdust promoted the production of high-molecular-weight components in polysaccharides and changed the monosaccharide composition of polysaccharides, as well as increased the mannose, glucuronic acid, and xylose contents of IPS. Tween-80 and oleic acid also changed the molecular weight distribution of polysaccharides but only slightly affected the composition of monosaccharides. The bioactivity assay indicated that the polysaccharides obtained by adding corn stalk possessed high hydroxyl radical scavenging and antitumor activities. The effect of poplar sawdust was slightly weaker than that of corn stalk. EPS and IPS obtained from a culture with Tween-80 and oleic acid possessed low antioxidant activity. Moreover, their antitumor activity was improved and lost, respectively. The results obtained in this work are useful for improving the understanding of the optimization and regulation of bioactive polysaccharide production in the submerged culture of B. fumosa.