The conicity index (CI), an anthropometric parameter of visceral obesity, has exhibited a strong correlation with the incidence of atherosclerosis and cardiovascular events. This study aimed to explore the relationship between CI and carotid plaque instability, as well as the potential differences between sexes in this context.Our cross-sectional study included a total of 44,248 participants from the China Stroke High-risk Population Screening and Intervention Program (CSHPSIP), all of whom underwent carotid ultrasound for the evaluation of carotid plaque. Multivariate logistics regression analysis and restricted cubic spline (RCS) curves were employed to examine the association between CI and the prevalence of carotid plaques and unstable plaques. A subgroup analysis was conducted to account for potential confounding variables.Patients belonging to the higher CI quartiles had a greater prevalence of carotid plaque and unstable carotid plaque. After adjusting for confounding factors, each standard deviation (SD) increase in CI was associated with an odds ratio (OR) of 1.06 (95% confidence interval: 1.03-1.08) for carotid plaque and 1.03 (95% confidence interval: 1.00-1.05) for unstable carotid plaque. The RCS analysis revealed an escalating trend in the prevalence of unstable carotid plaque with increasing CI values. However, this observed trend was not evident among female participants. Furthermore, subgroup analysis revealed a more pronounced correlation between CI and carotid plaque instability in individuals with a fasting blood glucose (FBG) of ≥ 7 mmol/L and an unhealthy lifestyle characterized by physical inactivity and current smoking.Our findings demonstrated that CI was significantly associated with carotid plaque and recommend CI as a promising indicator for the initial screening of atherosclerotic plaques in the future.