PURPOSE: Colorectal cancer (CRC) is among the most common malignant tumors worldwide, posing a significant threat to human health. Most patients with CRC are refractory to existing treatment regimens, such as immune checkpoint blockades (ICBs), yielding unsatisfactory outcomes. This study aimed to explore the effect and mechanism of a natural product, indole-3-carbinol (I3C), in CRC pathogenesis and immunotherapy. METHODS: A series of in vitro experiments, such as the cell counting kit-8 and wound healing assays, were used to assess the proliferative, colony-formating and migratory capacity of human CRC cells after I3C treatment. In vivo experiment, xenograft growth assay was conducted to verify the effect of I3C on CRC. Hematoxylin-eosin (HE) staining was utilized to evaluated the toxic effect of I3C. Immunohistochemical staining was used to detect CD8 RESULTS: I3C significantly inhibited CRC cell proliferation, colony formation, and migration capacity in vitro and in vivo. The result of HE staining indicated that I3C exert no significant toxic effect on heart, liver and kidney. HPA data analysis and RT-qPCR results demonstrated that PTEN expression was lower in CRC tissues than in normal tissues or cells. Besides, I3C exerted antitumor activity and promoted CD8 CONCLUSIONS: These findings suggested that by upregulating PTEN expression, the natural product I3C strongly prevented tumor progression and exerted no systematic toxicity in the major organs such as heart, kidney and liver. Furthermore, I3C significantly enhanced PD1ab therapeutic effect in CRC, highlighting its role as a candidate preventive or therapeutic compound for CRC therapy, especially in combination with PD1ab therapy. Further clinical trial should be conducted in the future.