Objective To observe the effect of modified Changpu Yujin Decoction on the blood ammonia level and expression of GFAP and AQP4 in the brain of rats with acute-on-chronic liver failure.Methods The Wistar rats were randomly divided into normal group,model group and modified Changpu Yujin Decoction group（administered group）.The model of acute-on-chronic liver failure was established by acute attack of human serum albumin combined with D-GalN/LPS.After the modeling was completed,the rats in administered group were treated with modified Changpu Yujin Decoction for 10 days.The rats in the model group and control group were given 0.9% normal saline for 10 days.ALT,AST and TBIL of each group were tested;the arterial blood ammonia level was detected by glutamate dehydrogenase rate method.The pathological changes of liver tissues were observed by HE staining.The ultrastructural changes of brain tissues were observed by transmission electron microscopy.The expression of GFAP and AQP4 in cortex of rats were detected by immunofluorescence staining.Results ALT,AST and TBIL,as well as the arterial blood ammonia level of modified Changpu Yujin Decoction group were reduced（P<0.05）,the astrocyte edema improved,the expression of AQP4 decreased（P<0.05）and the expression of GFAP increased（P <0.05）.Conclusion The modified Changpu Yujin Decoction could improve liver damage and reduce the blood ammonia level,regulate the expression of AQP4 and improve the astrocyte edema in rats with acute-on-chronic liver failure.