Objective To analyze the plasma interleukin 33 (IL-33) changes in sepsis of different severity. Methods A total of 120 sepsis patients who were admitted to the Intensive Care Unit (ICU) in our hospital  from January 2012 to April 2015 were selected. According to organ function of the patients and the severity of infection they were divided into general sepsis group (36 cases), severe sepsis group (36 cases) and septic shock group (48 cases). And 120 volunteers for health check during the same period were selected as control group. APACHE Ⅱ score, procalcitonin (PCT), IL-33, IL-6, IL-1β, changes in tumor necrosis factor-α (TNF-α) and sST2 of the patients were compared at different time in the ICU. The duration of ICU stay and mechanical ventilation in the ICU were compared between the sepsis groups. The changes of IL-33 levels were compared between the sepsis patients with different prognosis at different time. Results APACHE Ⅱ score, PCT and IL-33 levels in the sepsis groups were higher than those in the control group; and APACHE Ⅱ score, PCT and IL-33 levels in the sepsis patients increased with the increase in the severity of the disease, the difference was statistically significant (P < 0.05). The duration of ICU stay and mechanical ventilation in the sepsis patients prolonged as the disease worsened, and mortality of the septic shock group was significantly higher than that of other two groups (P < 0.05). APACHE Ⅱ score, PCT, IL-33, IL-6, IL-1β, TNF-α and sST2 levels in the sepsis groups were significantly different from those in the control group (P < 0.05). APACHE Ⅱ score, PCT, IL-33 and IL-6 levels in the sepsis patients decreased as the duration of ICU prolonged, the differences were statistically significant (P < 0.05). IL-1β, TNF-α and sST2 levels in the sepsis patients did not have significant changes as the duration of ICU stay prolonged (P > 0.05). Nine patients who died 30 d after hospitalization in the ICU were enrolled into the death group, and the remaining 111 survival patients into the survival group. IL-33 level of the death group rose as the ICU time prolonged, but the difference was not statistically significant (P > 0.05). IL-33 level in the survival group decreased as the duration of ICU stay prolonged. IL-33 level in the death group was lower than that in the survival group within 3 h in the ICU, but significantly higher than that in the survival group at 24 h and on the 5th day in the ICU (P < 0.05). Conclusions In patients with sepsis, plasma IL-33 level increases at the time of entering into ICU, but reduces as the duration of ICU stay prolongs, and elevates as the patients' situation worsens. Therefore, plasma IL-33 level can be a prognostic predictor for sepsis patients.