Various carbon sources affect the growth of theG. lucidumfruiting body, and the cassava stalk is considered a promising carbon source forG. lucidum. The composition, functional group characteristics, molecular weight distribution, antioxidant activity in vitro, and growth effect ofL. rhamnosusLGG ofG. lucidumpolysaccharides (GLPs) under cassava stalk stress were investigated by gas chromatography-mass spectrometry, near-infrared spectroscopy, and gel chromatography. The results showed that GLPs consisted of D-glucose, D-galactose, and seven other monosaccharides. The end of the sugar chain had β-D-Glc and β-D-Gal configurations. The total sugar content in GLP1 was the highest (4.07%), and GLP1, GLP2, GLP3, and GLP5 had the β-D-Gal configuration, while GLP4 and GLP6 had the β-D-Glc configuration. The greater the proportion of cassava stalk, the greater the maximum molecular weight of GLPs. The total antioxidant capacities of GLPs obtained from different cassava stalks significantly varied, as well as their stimulating effects on theL. rhamnosusLGG growth. Higher concentrations of GLPs corresponded to the more intensive growth ofL. rhamnosusLGG. This study provided essential data support for cassava stalk as a carbon source inG. lucidumcultivation.