BACKGROUND: Cardio-renal-metabolic (CRM) multimorbidity, defined by the co-occurrence of cardiovascular disease (CVD), chronic kidney disease (CKD), and type 2 diabetes (T2DM), poses a growing burden for multimorbidity management. The atherogenic index of plasma (AIP), a lipid-based marker of insulin resistance, may help predict CRM progression, but its prognostic value remains unclear. METHODS: We used data from 5805 adults free of CRM at baseline in the China Health and Retirement Longitudinal Study. AIP was calculated as log10[triglyceride (TG)/high-density lipoprotein cholesterol (HDL-C)]. CRM onset was defined as incident CVD, T2DM, or CKD. Multistate models captured transitions from no disease to single, dual, and triple CRM states. Cox regression and time-dependent receiver operating characteristic (ROC) curves were used to assess associations and predictive performance. RESULTS: AIP showed a clear dose-response relationship with CRM progression. Each 1-standard deviation (SD) increase in AIP was associated with higher risks of single (HR 1.17, 95% CI 1.13-1.22), dual (HR 1.31, 95% CI 1.21-1.41), and triple CRM (HR 1.57, 95% CI 1.27-1.94) diseases. Strong associations were observed for new-onset T2DM and stroke, but not CKD. AIP yielded the highest discrimination for triple CRM (5-year area under the curve [AUC] = 0.710). Multistate modeling showed each 1-SD increase in AIP raised the hazard of transitioning from single to dual CRM by 14% and from dual to triple by 31%. CONCLUSIONS: AIP is an independent, simple, and low-cost predictor of CRM onset and progression, with potential utility for early risk stratification and prevention.