Background Immunologic derangement may be the critical pathophysiologic mechanism in sepsis, and immunotherapy might be a potential new treatment. Si-ni-tang (SNT), an ancient Chinese herbal formula documented in Shanghan Lun, has been used for treating severe sepsis for thousands of years. Research shows that it may have a therapeutic benefit for sepsis. This study will evaluate the feasibility of testing the effects of SNT on immune function in sepsis patients. Methods/design This is a pilot randomized controlled study. Eligible sepsis patients admitted to our medical intensive care unit will be randomly allocated to the control group or the SNT group. Both groups will receive standard therapy according to the recommendations of the Surviving Sepsis Campaign. In addition, the SNT group will receive SNT (150 mL per day for 3 days) orally or by gastric tube, while the control group will receive 150 mL of normal saline. The primary outcome is to assess the feasibility of this treatment. The secondary outcomes include: (1) immune function measured by monocyte human leukocyte antigen-DR (mHLA-DR) expression, procalcitonin, and the ratio of CD4+ to CD8+ T lymphocytes and (2) other clinical data, such as the 28-day all-cause mortality, Sequential Organ Failure Assessment (SOFA) scores, Acute Physiology and Chronic Health Evaluation (APACHE) II scores, both of the latter on days 0 and 3. Discussion This study aims to evaluate the feasibility of testing the efficacy of SNT for treating sepsis when used as an adjunctive treatment with the standard therapy recommended by the Surviving Sepsis Campaign.