Objective To investigate the effects of chronic heart failure (CHF) with comorbid type 2 diabetes mellitus (T2DM) on cognitive function in elderly. Methods A total of 116 elderly patients with CFH and (or) T2DM were selected, who underwent inpatient rehabilitation in Jiangsu Rongjun Hospital and home-based rehabilitation in Liangxi District of Wuxi City from October 2019 to May 2020. They were divided into three groups according to the comorbidity of CHF and T2DM:CHF group (n=47), T2DM group (n=29), and CHF-T2DM group (n=40). General data of the patients were collected, and their cognitive function was assessed using the Montreal Cognitive Assessment Scale (MoCA). Based on MoCA score (< 26), the included 116 patients were classified as having cognitive impairment (CI, n=55) and having no cognitive impairment (NCI, n=61). Statistical analyses were performed using SPSS statistics 25.0. Logistic regression was used to analyze the affecting factors of CI in CHF patients with comorbid T2DM. Results The CHF-T2DM group had a significantly lower MoCA score and a significantly higher incidence of CI than the CHF and T2DM groups (P<0.05 for both). The CI group had significantly lower left ventricular ejection fraction (LVEF) and significantly higher score than the NCI group in smoking, course of CHF and T2DM, family history of dementia, CHF and T2DM, comorbid hypertension, comorbid atrial fibrillation, systolic blood pressure, glycosylated hemoglobin Alc (HbA1c), urea nitrogen, NT-proBNP, Hamilton depression scale score, and Pittsburgh sleep quality index score (all P<0.05). Multivariate logistic regression analysis showed that course of CHF, family history of dementia, comorbid atrial fibrillation, glycated hemoglobin, amino terminal pro-brain natriuretic peptide, and left ventricular ejection fraction were independent risk factors for the development of CI in CHF patients with T2DM (P<0.05 for all). Conclusion Compared with T2DM alone and CHF alone, CHF with comorbid T2DM would aggravate CI in the elderly.