Abstract: Objective: To investigate the association between cardiometabolic index(CMI) and new-onset arthritis among middle-aged and older adults in China, providing a novel metabolic perspective for early risk identification. Methods: Based on the prospective cohort data from China Health and Retirement Longitudinal Study(CHARLS), participants aged ≥ 45 years without arthritis at baseline were included. Associations were assessed using multivariable logistic regression, restricted cubic spline(RCS) regression models, piecewise regression models and subgroup analyses. Results: A total of 4,642 participants were included, with a median age of 58.0(51.0-65.0) years, among whom 1, 475 participants were identified to be new-onset arthritis cases during the follow-up. After multivariable adjustment, compared with the lowest quartile(Q1) of CMI, the Q2 group(OR=1.25, 95% CI: 1.05-1.50) and the Q3 group(OR=1.29, 95% CI: 1.08-1.56, P=0.007) showed a significantly increased arthritis risk, and the Q4 group also exhibited an elevated trend(OR=1.29, 95% CI: 1.06-1.58, P=0.013). RCS analysis revealed a nonlinear "inverted J-shaped" association between CMI and arthritis risk(nonlinearity P=0.001), and piecewise regression identified a breakpoint of 1.709(95% CI: 1.632-1.786). No significant interactions were observed in subgroup analyses(P>0.05). Conclusion: Elevated CMI levels are associated with an increased risk of new-onset arthritis among middle-aged and older adults in China, exhibiting a nonlinear "inverted J-shaped" association. CMI may serve as a simple metabolic biomarker for assessing arthritis risk.