【Objective】 To explore the effects of periodontitis on the brains of Alzheimer’s disease (AD) mice and the effects of N-acetyl-L-cysteine (NAC) on the periodontium and the brain of AD mice with experimental chronic periodontitis (CP). 【Methods】 Ten wild-type C57 mice were in the blank control group (Group C57), and another 40 3-month-old APP/PS1 transgenic mice were randomly divided into four groups: AD+CP group, AD+CP+NAC group, AD+NAC group, and AD group. The periodontitis model was established in mice in AD+CP group and AD+CP+NAC group by silk ligation. At the same time, mice in AD+CP+NAC group and AD+NAC group were injected with NAC (200 mg/kg). The height of alveolar bone loss was detected after 3 weeks, and the cell morphology of the hippocampus was observed. We determined the expression levels of NLRP3 inflammasome and amyloid-β (Aβ) in the hippocampus as well as the expression levels of interleukin (IL)-1β and IL-18 in the hippocampus and gums by ELISA. 【Results】 Compared with AD group, the height of alveolar bone loss in AD+CP group was higher (P<0.05), and the expression levels of IL-18 in the gum and IL-1β and NLRP3 in the hippocampus were increased significantly (P<0.05). In addition, compared with AD+CP group, the height of alveolar bone loss in AD+CP+NAC group was reduced significantly (P<0.05), the expression levels of IL-1β and IL-18 in the gum were reduced significantly (P<0.05) as well as the expression levels of IL-1β and Aβ in the hippocampus were reduced significantly (P<0.05). Compared with C57 group, the neurons in the hippocampus of AD+CP group were more loose and disordered, and the activation of microglia was increased, while the disarrangement of cells in AD+CP+NAC group was less than that in AD+CP group. 【Conclusion】 Periodontitis can aggravate brain inflammation in AD mice. NAC can effectively reduce alveolar bone resorption in AD mice caused by periodontitis as well as reduce the levels of inflammatory factors in the gum and hippocampus. NAC can effectively reduce the alveolar bone absorption of AD mice caused by periodontitis, reduce the level of inflammatory factors in the gingiva and hippocampus, and reduce the damage of nerve cells in the hippocampus.