Background Tradition treatment of sepsis and new therapies, including high dose corticosteroids and non-steroidal anti-inflammatory drugs, have proven unsuccessful in improving survival. This study aimed to evaluate the potential efficacy of immunomodulating therapy using Ulinastatin (UTI) plus Thymosin α1 (Tα1) for improving organ function and reducing mortality in patients with severe sepsis.Methods A prospective study was carried out with randomized and controlled clinical analysis of 114 patients conforming to the enrollment standard. All patients had severe sepsis and received standard supportive care and antimicrobial therapy. Fifty-nine patients were also administered UTI plus Tα1 (defined as Group A), 55 patients were given a placebo (defined as Group B). Clinical parameters were determined by evaluation with the Acute Physiology and Chronic Health Evaluation Ⅱ (APACHE Ⅱ), multiple organ failure (MOF) and the Glasgow Coma Scores (GCS) on entry and after therapy on the 3rd, 8th, and 28th day. By flow cytometery and ELISA lymphocyte subsets and cytokines were analyzed. Survival analysis was determined by the Kaplan-Meier method at 28, 60, and 90 days. Results Based on comparison of the two groups, patients in Group A exhibited a better performance in organ failure scores which was noticeable soon after initiation of treatment. Patients in Group A also demonstrated a better resolution of pre-existing organ failures during the observation period. After initiation of treatment, significant improvements in the CD4+/CD8+ ratio, a quicker balance between proinflammatory mediators such as tumor necrosis factor a, interleukin 6 and anti-inflammatory cytokines including interleukin 4 and interleukin 10 were found. This was followed by cumulative survival increases of 17.3% at 28 days, 28.9% at 60 days, and 31.4% at 90 days in Group A. The reduction in mortality was accompanied by a considerably shorter stay in the ICU and a shorter length of supportive ventilation, antimicrobial and dopamine therapy.Conclusion UTI plus Tα1 has a beneficial role in the treatment of severe sepsis.