We investigated the chemical constituents and anti-tumor activity of cultivated <i>Pholiota adiposa in vitro</i> using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry. HepG-2, A549, HeLa, and MCF-7 cells, which are 4 kinds of human cancer cell lines, were cultured<i> in vitro</i>, treated with different concentrations of the ethanol extract of<i> Ph. adiposa</i> (EPA), and cytotoxicity was determined using the cell counting kit-8 assay. Flow cytometry was used to analyze the apoptosis of HepG-2 cells via annexin V-fluorescein isothiocyanate/propidium iodide double staining. Expression levels of apoptosis-associated proteins were determined via Western blotting analysis. Thirty-five components were consistent with those recorded in the chemical composition database, with sterols, fatty acids, and polysaccharide compounds accounting for a relatively high proportion. EPA showed the strongest cytotoxicity against HepG-2 cells, increasing the apoptosis rate up to 23.71 ± 1.59% at a concentration of 50 μg/mL. <i>Ph. adiposa</i> has various functional chemical constituents and potential anti-tumor applications. We found that the functional constituents exerted anti-tumor activity by inducing apoptosis. Furthermore, the expression levels of BCL-2-associated X were increased, whereas those of BCL-2 were decreased in cells after treatment with EPA. These results suggest that EPA induces HepG-2 cell apoptosis via a caspase-mediated pathway.