Type 1 diabetes mellitus (T1DM) is a metabolic disease characterized by insulin deficiency and often accompanied by hypercholesterolemia. NAC is an effective antioxidative drug, but its application in the treatment of diabetes is still rare. A total of forty beagles were randomly divided into five groups: control group, DM group, INS group, INS with NAC group, and NAC group. The experiment lasted for 120 days. Results revealed that biochemical criterion increased in the DM group, while the indicators significantly decreased on the INS combined with NAC treatment group. Moreover, the insulin released test demonstrated that the model of T1DM was successfully constructed. The result of B ultrasound of gallbladder showed remarkable cholestasis in DM group. The cholesterol metabolism-related enzyme activity (HMGCR and SQLE) was evidently increased in DM group, but decreased in INS and NAC group. The content of TG, LDL-c, and HDL-c in liver was detected by the kit, and it was found that the content of TG, LDL-c, and HDL-c in DM group were reduced. Histopathological observation revealed that the cholestasis of liver cells and hepatic cords were disordered in DM group, the symptoms were alleviated under INS and NAC treatment. Additionally, the protein and mRNA expression of HMGCR and LDLR were obviously increased in DM group, but down regulated in INS and NAC treatment group. Overall, the liver function injury and secondary hypercholesterolemia can be found in T1DM canines, and NAC can relieve cholesterol metabolism disorder in the treatment of canine T1DM.