This study aimed to investigate the inhibitory roles of a polysaccharide extract from Inonotus obliquus on U251 human neurogliocytoma cells cultured in vitro. After administering the polysaccharide extract from I. obliquus to U251 cells cultivated in vitro, methyl thiazolyl tetrazoliym assay was performed to measure the inhibitory effects of the extract on tumor cell proliferation. The expression of the apoptosis-related proteins Bcl-2 and caspase-3 were determined by Western blotting. Different concentrations of I. obliquus extract (25, 50, 100, 200, and 500 µg/mL) were added to U251 cells at 24, 48, and 72 hours. Methyl thiazolyl tetrazoliym assay showed that the inhibition ratio increased with increased extract concentration and prolonged treatment duration. The I. obliquus extract sharply decreased the expression of Bcl-2 but dramatically increased the expression of caspase-3. This function was gradually enhanced with increased drug concentration and prolonged treatment duration. The I. obliquus extract can inhibit the proliferation of tumor cells. This inhibition function is closely related to the downregulation of Bcl-2 and the upregulation of caspase-3.