Objective: To observe the application of fMRI in detecting the default mode network (DMN) of patients with carotid artery stenosis and cognitive impairment. In the combination with neuropsychological scores, to explore the relationship between DMN and cognitive function under hypoperfusion. Methods: A sample of 32 individuals, who went to Beijing Tiantan Hospital affiliated to Capital Medical University for treatment since June 2017 to December 2018, have been selected for this research. Among them 16 patients were confirmed by DSA with severe stenosis (stenosis rate ≥ 70%) of unilateral internal carotid artery (ICA); while the other 16 were healthy people as normal controls. The neuropsychological tests [Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA)] and blood oxygen level-dependent functional MRI (BOLD-fMRI) were performed to all the individuals. Furthermore, the function imaging was performed to develop a brain network model and to compare the differences of functional connectivity (FC) between 2 groups. Results: The MoCA score in ICA stenosis group was lower than that in normal control group [(21.19 ± 4.00) score vs. (27.94 ± 2.00) score; t = 2.609, P = 0.048], indicating there was cognitive impairment in some degree in ICA stenosis. The significant FC in ICA stenosis group and normal control group were widely distributed between the frontal lobe and the occipital lobe, within the hemisphere and between both hemispheres. The FC of ICA stenosis group was significantly weaker than that of normal control group. The functional reduction between both hemispheres was indicated in the regions including the right frontal lobe, left parietal lobe and left temporal lobe, the lower margin of right parietal lobe and left amygdala. In the hemisphere, the FC was weaken between the frontal and prefrontal lobes. A specific enhancement in the function of the local brain region was also shown in ICA stenosis group. Conclusions: Regarding the patients with carotid artery stenosis and cognitive impairment, the DMN has changed. The enhancement or attenuation of the FC within the hemisphere and between both hemispheres were reflected on fMRI. This may be related to the local functional changes under long-term hypoperfusion. Copyright © 2019 by the Editorial Board of Chinese Journal of Contemporary Neurology and Neurosurgery.