Associations of physical function and multimorbidity with cognitive impairment in community-dwelling Chinese octogenarians

China’s rapidly aging population has intensified concerns about cognitive impairment and chronic disease burdens among older adults. Despite this, research on the interplay between physical function, multimorbidity, and their associations with cognitive impairment in community-dwelling octogenarians remains limited. This study investigates the associations between physical function, chronic disease prevalence, and cognitive impairment among home-dwelling adults aged ≥ 80 years in China, with a focus on demographic and clinical predictors of cognitive decline. A cross-sectional analysis of 13,027 octogenarians assessed physical function using the Barthel Index (BI) and cognitive status via the Mini-Cog. Logistic regression models evaluated the impact of demographic factors (age, gender, occupation), chronic diseases (hypertension, stroke), and psychosocial factors (aggression, depression) on cognitive impairment severity. Of the participants, 37.88% exhibited high functional independence (BI = 100). Cognitive impairment was significantly influenced by age, gender, and multimorbidity ( p < 0.01). Depression (OR 11.335; 95% CI 8.853–14.514) and aggression (OR 1.93; 95% CI 1.161–3.217) were strongly associated with cognitive impairment severity. Hypertension was moderately correlated with mild cognitive impairment (OR 1.495; 95% CI 1.285–1.739), while stroke tripled the odds of mild-to-moderate cognitive impairment (OR 3.301; 95% CI 2.131–5.113). Reduced self-care ability was associated with nearly fourfold higher odds of cognitive impairment ( p < 0.01). Age, occupation, and lifestyle factors synergistically are associated with chronic disease burden and cognitive impairment in China’s oldest-old population. These findings underscore the urgency of personalized healthcare strategies to preserve independence and mitigate cognitive decline in high-risk octogenarians.