Abstract: Objective To investigate the impact of mild-to-moderate frailty on the long-term prognosis of hospitalized elderly patients with type 2 diabetes mellitus (T2DM). Methods A retrospective cohort study was designed, which contains T2DM patients aged ≥65 years and hospitalized in the Department of Geriatrics at Peking Union Medical College Hospital (PUMCH) from 2014 to 2022. Frailty status of those T2DM patients was assessed using the Clinical Frailty Scale (CFS), and the data of comorbidities, functional status, nutritional indices, and geriatric syndromes were collected. The primary endpoint was all-cause mortality, with secondary endpoints including rehospitalization rate and severe disability. Cox proportional hazards regression models were employed to analyze the association between mild-to-moderate frailty and outcomes. Results A total of 367 elderly T2DM patients were enrolled, comprising 164 males (44.7%) and 203 females (55.3%), with an age ranging from 65 to 93 years (median age 74 years). According to the CFS assessment, 115 patients (31.3%) were identified as mild-to-moderate frailty (including 56 with mild frailty and 59 with moderate frailty). During a follow-up period of 2.3-10.3 years (median 5.6 years), the frail group exhibited significantly higher rates of severe disability, unscheduled rehospitalization, and all-cause mortality compared to the non-frail group. Multivariable Cox regression analysis revealed that mild-to-moderate frailty was an independent risk factor for severe disability (HR=4.82, 95% CI1.57-14.88, P=0.006). Conclusion The presence of mild-to-moderate frailty significantly increases the risk of long-term adverse outcomes. Clinical practice is recommended to strengthen frailty screening and comprehensive intervention for elderly T2DM patients to improve their quality of life and clinical outcomes.