Objective: To study the expression of pathogenesis of CD4+CD25+Foxp3+ regulatory T cells in HBV infection in acute –on-chronic liver failure(ACLF) patients. Methods Test CD4+CD25+Foxp3+ regulatory T cells level in peripheral blood of 36 patients with ACLF, 38 patients with chronic hepatitis B , 40 cases of asymptomatic carriers and 40 healthy people as control by flow cytometry method. Compare the frequency of CD4+CD25+Foxp3+ regulatory T cells in non recovery group and improved group of ACLF patients. Results the expression level of CD4+CD25+Foxp3+ regulatory T cells in peripheral blood among ACLF group, chronic hepatitis B (CAH), asymptomatic carrying group (ASC) and healthy control group was different. Regulatory T cells’ percentage was 5.14±1.03%,9.86±1.72%,7.93±1.67%,7.06±1.61%. The frequency of regulatory T cells of ACLF group was significantly lower than the CAH group, also lower than the ASC group and the healthy control group (P <0.01). The percentage of peripheral blood regulatory T cells of CAH group , ASC group and control group was different significantly (P <0.05). The percentage of peripheral blood regulatory T cells of ASC group and the healthy control group was no significant different (P> 0.05). The percentage of CD4+CD25+Foxp3+ regulatory T cells in study group was positively correlated with serum HBVDNA. Levels of CD4+CD25+Foxp3+ regulatory T cells showed no significant difference between non recovery group and improved group of ACLF patients (P> 0.05). Conclusion The low expression level of CD4+CD25+Foxp3+ regulatory T cells may be related to ACLF patients related with HBV. The level of regulatory T cells couldn’t indicate the outcome of ACLF patients.