Abstract Background Hepatitis B virus-related acute-on-chronic liver failure (HBV-ACLF) is the most prevalent type of ACLF in China. The mortality rate of HBV-ACLF has decreased in recent years due to advances in treatment therapies; however, it is still above 50%. Many cases of HBV-ACLF are caused by HBV reactivation due to discontinuation of nucleoside analog treatment. The present study focused on plasma levels of superoxide dismutase (SOD) in HBV-ACLF patients and investigated whether the plasma level of SOD is a useful biomarker in assessing disease severity and predicting outcomes of HBV-ACLF patients, including patients treated with Entecavir (ETV) and patients who were withdrawn from ETV treatment. Methods Plasma samples and clinical data from 200 HBV-ACLF patients and from age- and sex-matched cirrhotic and healthy controls were collected and analyzed. Plasma levels of SOD were measured using an ELISA commercial kit. Results Among the HBV-ACLF patients, in the ETV withdrawal group, the mortality rate was higher than in the ETV group (69.95% vs 46.71%, P  < 0.05). Moreover, HBV-DNA and SOD plasma levels were higher in the ETV withdrawal group than in the ETV group ( Log 10 (HBV-DNA) : 6.49 ± 0.24 vs 4.79 ± 0.14, P  < 0.01; SOD: 463.1 ± 27.61 U/mL vs 397.2 ± 10.97 U/mL, P  < 0.05). The mortality and liver transplantation rates were significantly higher in HBV-ACLF patients with plasma levels of SOD > 428 U/mL than in patients with plasma SOD levels ≤ 428 U/mL. Conclusions Reactivation of HBV and elevated oxidative stress caused by discontinuation of ETV treatment are crucial factors in the pathogenesis of HBV-ACLF. Plasma level of SOD may serve as a useful biomarker in estimating disease severity and predicting outcomes of HBV-ACLF patients who stop ETV treatment.