Recent clinical trials have shown that combining chemotherapy with anti-angiogenic therapy and immunotherapy can enhance survival outcomes for patients with advanced colorectal cancer (CRC). However, the underlying mechanisms remain unclear. To address this knowledge gap, we investigated the effects and potential mechanisms of combining oxaliplatin with the anti-angiogenic drug fruquintinib and a PD-1 monoclonal antibody. Our findings indicate that this combination improves vascular conditions within the tumor microenvironment (TME), thereby downregulating the expression of hypoxia inducible factor-1α (HIF-1α), and alleviating tumor hypoxia. Moreover, the inclusion of oxaliplatin activates the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway in the TME, and further increases the proportion of cytotoxic T cells, dendritic cells (DC) and natural killer (NK) cells in the TME without elevating immunosuppressive cells, such as regulatory T cells (Tregs) and M2-type macrophages, thus creating a more immunoreactive microenvironment that effectively inhibits colon tumor growth. Importantly, these results provide a theoretical basis for the clinical application of this three-agent regimen and offer new insights into combination therapy strategies for CRC treatment.