Objective To investigate energy metabolism with mitochondrial damage through building acute on chronic liver failure (ACLF) rat model.Methods Male SD rats were randomly divided into three groups,both ACLF and NAC group had 15 rats in each,control group had 10 rats.Rat model was treated with antioxidant therapy.Cytochrome C (Cyt C) was determined by using ELLSA and Cyt C,and fluorescence microplate method was used to determine mitochondrial permeability transition pore (MPTP) relative fluorescence unit (RFU) ; ELISA method was used to determine liver tissue and plasma glutathione (GSH),malonaldehyde (MDA),and superoxide dismutase (SOD).Results Levels of MDA in both liver and plasma are higher than that in normal and NAC groups (P ＜0.05) ; Levels of MDA in NAC group was higher than in normal group (P ＜ 0.05) ; levels of GSH and SOD of plasma in ACLF group is lower than in normal and NAC group (P ＜ 0.05) ; Cyt C in the normal group was the highest and ACLF group the lowest (P ＜0.05) ; ACLF group had the highest MPTP and normal group the lowest; MPTP with Cyt C and ATP content was negatively correlated (r =-0.858,-0.799) ; Cyt C and ATP content was positively correlated (r =0.78).Conclusion There is oxidative stress in ACLF rats; the flowing of Cyt C may be one of reasons of abnormal energy metabolism in ACLF; exogenous supply of NAC can improve the antioxidant capacity of the liver cells.