Objective: Based on the "phlegm-dampness" theory of Traditional Chinese Medicine (TCM), this longitudinal study utilized data from the China Health and Retirement Longitudinal Study (CHARLS) to investigate the association between a novel nutritional index—triglyceride-total cholesterol-body mass index (TCBI)—and the risk of diabetes in middle-aged and older adults. Methods: Data were obtained from the CHARLS database (2011–2020). Cox regression was employed to examine the longitudinal association between TCBI and diabetes risk. Kaplan-Meier curves, restricted cubic splines, receiver operating characteristic (ROC) curves, and subgroup analyses were used for validation. Results: A total of 8,765 Chinese participants aged ≥45 years were included in this study. Cox regression analysis revealed that, after adjusting for covariates, TCBI was longitudinally associated with diabetes risk (HR: 1.38, 95% CI: 1.29–1.48).This finding aligns with the TCM "phlegm-dampness" theory, which posits that "internal accumulation of turbid phlegm and spleen dysfunction" contribute to the development of wasting-thirst disorder (diabetes). Restricted cubic spline analysis indicated a linear relationship between TCBI exposure levels and diabetes risk (P < 0.001). Kaplan-Meier curves demonstrated that individuals with higher TCBI levels had a greater cumulative incidence of diabetes. Subgroup analyses revealed interactions between TCBI and both marital status and history of hypertension (P < 0.05). ROC curve analysis showed that TCBI had a predictive efficacy of 0.64 for diabetes in the middle-aged and older population. Conclusion: TCBI level exhibits a significant positive linear association with diabetes risk in middle-aged and older adults, providing modern epidemiological support for the TCM theory that "phlegm-dampness causes wasting-thirst disorder."