Objective To determine the effect of hypertension complicated with diabetes mellitus (DM) on cognitive function in the elderly. Methods A total of 129 elderly patients (aged 60 to 80 years) admitted in our department from June 2015 to June 2016 were recruited in this study. They were divided into hypertension group (n=59), DM group (n=30), and hypertension complicated with DM group (hypertension+DM, n=40). Their general information and clinical data were collected. Their cognitive function was evaluated by mini-mental state examination (MMSE) and Montreal cognitive assessment (MoCA). Results Compared with hypertension group and DM group, the hypertension+DM group had lower scores of MMSE and MoCA, and higher incidence of mild cognitive impairment (MCI, all P<0.05). Logistic regression analysis indicated that age, glycosylated hemoglobin (HbA1c), duration of hypertension, solitary and familial history of dementia were risk factors for cognitive impairment (P<0.05), while high educational level was a protective factor (P<0.05). Conclusion Compared with essential hypertension and simple DM, hypertension complicated with DM aggravates cognitive impairment in the elderly.