Objective To investigate the efficacy and safety of Xihuang capsules as an adjuvant treatment for metastatic colorectal cancer and their impact on immune function. Methods A retrospective analysis was conducted on clinical data from 112 patients diagnosed with metastatic colorectal cancer. The patients were categorized into two groups: a control group (n=56) that did not take Xihuang capsules and an observation group (n=56) that did. The efficacy, improvement of quality of life, toxic and side effects and immune function of the two groups were analyzed and compared. Results After treatment, the disease control rate (DCR) and the rate of improvement in quality of life were significantly higher in the observation group compared to the control group. Additionally, levels of carcinoembryonic antigen (CEA) and the incidence of adverse reactions, including bone marrow suppression and liver and kidney function damage, were significantly lower in the observation group. Furthermore, the percentages of CD4+ and CD8+ T cells, the CD8+/CD4+ T cells ratio, as well as serum levels of high mobility group box-1 (HMGB1) and interleukin 2 (IL-2) in observation group were significantly elevated compared to pre-treatment levels. Subgroup analysis revealed that patients with a Karnofsky Performance Status (KPS) score ≤80, a high CD8+/CD4+ T cells ratio, and elevated HMGB1 levels experienced a significantly higher objective response rate (ORR) in the observation group. Conversely, patients with stage IVB disease, who had KPS score ≤80, a low CD8+/CD4+ T cells ratio and high CEA and IL-2 levels demonstrated a more pronounced DCR in the observation group. Conclusion Xihuang capsules exhibit promising clinical efficacy as an adjuvant treatment for advanced colorectal cancer. They not only enhance patients' quality of life and reduce the toxic and adverse effects of chemotherapy, but also improve immune function. These benefits are particularly significant in patients with a high tumor burden, indicating that Xihuang capsules are worthy of clinical application.