Objective: To investigate the expression and significance of polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) in patients with HBV-related acute-on-chronic liver failure (HBV-ACLF). Methods: Forty HBV-ACLF patients admitted to Yijishan Hospital of Wannan Medical College from September 2022 to August 2023, were selected as the study group. Twenty chronic hepatitis B (CHB) patients and 10 healthy control (HC) who were treated or underwent physical examination in this hospital during the same period, were selected as the control groups. The frequency of PMN-MDSC at admission was detected by flow cytometry and was compared in the study group and the two control groups. Spearman test was used to investigate the correlation between the frequency of PMN-MDSC and inflammation indexes or disease severity. HBV-ACLF patients were further grouped according to having or developing infection or not and patient prognosis at day 28 from inclusion, with the PMN-MDSC frequency compared. Results: The frequency of PMN-MDSC in HBV-ACLF group was higher than that in CHB group and HC group [3.77 (1.35, 5.11) % vs. 0.67 (0.55, 0.82) % / 0.50 (0.40, 0.59) %, all P < 0.001], whereas there was no statistical difference in the PMN-MDSC frequency between the control groups (P > 0.05). The frequency of PMN-MDSC in HBV-ACLF patients was positively correlated with the white blood cell count, the neutrophil lymphocyte ratio, the procalcitonin, the international normalized ratio, the total bilirubin, the Child-Turcotte-Pugh score, and the Model of End-Stage Liver Disease score (r = 0.347, 0.799, 0.506, 0.450, 0.462, 0.470, and 0.481, all P < 0.05), and was negatively correlated with the lymphocyte count ( r= -0.428, P＜0.01). In the 40 HBV-ACLF cases, 18 were complicated with bacterial or fungal infection at admission, 17 developed secondary infection during hospitalization, and 21 had poor 28-day prognosis, with significant higher PMN-MDSC frequencies compared with the controls (all P < 0.001). Conclusion: There is enrichment of PMN-MDSC in HBV-ACLF patients. The frequency of PMN-MDSC in peripheral blood of HBV-ACLF patients is closely related to infection risk, disease severity, and patient short-term prognosis.