Background: Despite tremendous efforts that have been made, cancer is still the leading cause of death all over the world. Chemotherapy, considered a routine method, always faces severe side effects and drug resistance. Cordyceps militaris (C. militaris) is a kind of folk tonic food and traditional Chinese medicine and was reported to have anticancer capacity. Since it is difficult to cure cancer via chemotherapy, preventing or inhibiting malignant cells by diet behavior seems useful and attractive. Objectives: In this study, we aim to assess the anticancer capacity of C. militaris polysaccharide (CMPs) and explore its anticancer mechanism. Materials and Methods: Polysaccharide was extracted by water from C. militaris. Its total sugar, protein percentage, and monosaccharide composition were measured via the phenol-sulfuric acid method, Bradford kit, and GC assay, respectively. The anti-proliferation effect of the CMPs was screened against several cancer cell lines by CCK8. Its anticancer effect was further studied by cell morphology, live/dead cell staining, and cell apoptosis study. Cellular reactive oxygen species (ROS) evaluation and western blots assay were conducted to explore its anticancer mechanism. Results: According to our data, the CMPs could effectively inhibit the proliferation of cancer cells, with IC50 values ranging from 437.8 μg/mL to 545.1 μg/mL. Administration of CMPs could cause morphological change among cells and induce cell apoptosis. The study mechanism revealed that the CMPs exerted an anticancer effect via increasing the cellular ROS level and activating the endogenous apoptosis pathway. Conclusion: The CMPs can effectively inhibit cancer cells via arousing cellular ROS and activating the endogenous apoptosis pathway.