Background: To explore the alteration of pattens of anatomical and functional connectivity (FC) of posterior cingulate cortex (PCC) in Parkinson's disease (PD) patients with cognitive dysfunction and the relationship between the connection strengths and cognitive state.Methods: We prospectively enrolled 20 PD patients with mild cognitive impairment (PD-MCI), 13 PD patients with normal cognition (PD-NC) and 13 healthy controls (HCs). All subjects underwent clinical evaluations and MRI scans. By collecting, preprocessing and FC analyzing resting-state functional magnetic resonance imaging (rs-fMRI) data, we extracted default mode network (DMN) patterns, compared the differences in DMN between the three groups and the analyzed the correlation between FC value with the commonly used neuropsychological testing.Results: There were not significant differences with regard to demographic data among the three groups. The PD-MCI showed significant worse performances in general cognition, and PD-NC and HCs showed comparable performances of cognitive function. Cognitive-related differences in DMN were detected in the bilateral precuneus (BPcu). Compared with the HCs, PD-NC and PD-MCI showed significantly decreased FC within BPcu (both P < 0.001). For PD-MCI, the rho of the the Fisher’s Z-transformed FC (zFC) value within BPcu with the TMTA, DSST and CFT-20min were − 0.50, 0.66 and 0.47, respectively. For PD-NC, the rho of the zFC value within BPcu with the MMSE was 0.58.Conclusions: Our research found that BPcu was the cognitive related region in DMN. As cognition declines, FC within BPcu weaken. For PD-MCI, the higher the FC values within BPcu were related to the better the performances of TMTA, DSST and CFT-20 min DR. For PD patients with normal cognition, the FC within BPcu were positively correlated with scores of MMSE.