OBJECTIVES: This study aimed to investigate the independent and interactive associations of social isolation and a healthy lifestyle on intrinsic capacity (IC) among older adults in China. DESIGN AND SETTING: A longitudinal cohort study was conducted using data from the China Health and Retirement Longitudinal Study (CHARLS), spanning three waves (2011, 2013, and 2015) and involving adults aged 60 years and older. PARTICIPANTS: A total of 4495 older adults with complete data on IC, social isolation, healthy lifestyle, and relevant covariates at baseline were included in the final analysis. MEASUREMENTS: IC was assessed using a composite score across five domains: locomotor, cognitive, sensory, vitality, and psychological. Social isolation was measured using an index based on living arrangements, marital status, contact with children, and social participation. A healthy lifestyle was defined based on five factors: smoking, alcohol consumption, physical activity, sleep duration, and body mass index (BMI). Multiple linear regression and linear mixed-effects models were used to examine cross-sectional and longitudinal associations. Interactive terms between social isolation and a healthy lifestyle were analyzed. Sensitivity analyses were performed by stratifying participants by age and gender. RESULTS: At baseline, 31.0%, 62.3%, and 6.7% of participants had social isolation scores of 0, 1-2, and 3-4, respectively, while 28.5%, 37.0%, and 34.6% had healthy lifestyle scores of 0-2, 3, and 4-5, respectively. Compared to participants with no social isolation (score = 0), the IC scores decreased for participants with social isolation scores of 1-2 and 3-4 (β = -0.36, 95% confidence interval [CI]: -0.42 to -0.30) and (β = -0.65, 95% CI: -0.77 to -0.53), respectively. In contrast, adherence to a healthy lifestyle was associated with improved IC (β = 0.27, 95% CI: 0.20 to 0.34 for score 3; β = 0.54, 95% CI: 0.47 to 0.60 for score 4-5). Interactive analysis showed that a healthy lifestyle mitigated the adverse associations of social isolation on IC, but this protective effect weakened as social isolation increased. Sensitivity analyses confirmed the robustness of these findings. CONCLUSION: This study identified significant associations between social isolation, healthy lifestyle, and intrinsic capacity in older Chinese adults. Social isolation was negatively associated with IC, while a healthy lifestyle was positively associated with IC. The observed interaction between social isolation and healthy lifestyle highlights the importance of integrated interventions targeting both social engagement and health behaviors in supporting IC during aging.