As global aging accelerates, frailty has emerged as a critical public health challenge. Although social isolation is recognized as a prevalent psychosocial risk factor among older adults, the longitudinal mechanisms linking it to frailty—specifically via the chain mediating pathways of depression and cognitive decline—remain underexplored.

This study aimed to investigate the presence and magnitude of the “social isolation → depression → cognitive function → frailty” chain mediation pathway in the Chinese elderly population.

Using four waves of data (2011–2018) over 7 years from the China Health and Retirement Longitudinal Study (CHARLS), we analyzed a final sample of 3316 participants aged ≥60 years who were non-frail at baseline. The chain mediation pathways were tested by integrating logistic regression analyses within a structural equation modeling framework. The significance of direct and indirect effects was examined using the bias-corrected bootstrap method with 5000 resamples.

Among the 3316 participants (mean age: 68.9 ± 5.9 years; 60.5% female), 1947 (58.7%) developed frailty during follow-up. After full adjustment for covariates, social isolation exhibited no significant direct effect on frailty. Instead, its impact was exclusively indirect through three distinct pathways: (1) the independent mediating route of depression (effect value = 0.976; 95% CI: 0.200–1.735); (2) the independent mediating route of cognitive function (effect value = 0.490; 95% CI: 0.003–0.985); and (3) the chain mediating route from depression to cognitive function (effect value = 0.173; 95% CI: 0.035–0.311). The total indirect effect value was 1.638 (95% CI: 0.637–2.611), confirming a full mediation mechanism.

Social isolation functions as a distal determinant that indirectly escalates the risk of frailty by potentially inducing depression, which subsequently accelerates cognitive decline. These findings highlight the need for integrated interventions targeting social engagement, mental health, and cognitive function to prevent or delay frailty in older populations.