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Introduction Oral diseases affect nearly half of the global population and impose substantial health and economic burdens, particularly in China. However, evidence on the cost-effectiveness of preventive oral health interventions remains limited. This study aims to evaluate the long-term cost-effectiveness of promoting population-based oral self-care behaviours across China.Methods We developed the China Oral Health Model, a prevention-oriented population policy simulation that projects annual transitions across major oral health states (dental caries, periodontitis, tooth loss and edentulism) among individuals aged ≥12 years. Preventive self-care strategies were modelled as relative reductions in the probabilities of disease onset and progression, implemented through changes in intermediate clinical states. The model was parameterised using nationally representative data from the Fourth National Oral Health Survey of China, 2015 to 2017, supplemented by age-specific prevalence estimates from the literature. We evaluated three key oral self-care behaviours: brushing teeth twice daily, using dental floss and using mouthwash. Intervention effect parameters for brushing, flossing and mouthwash were obtained from published clinical and epidemiological studies, with uncertainty assessed in sensitivity analyses. We conducted a societal-perspective cost-effectiveness analysis, stratified by rural and urban populations, including intervention costs estimated from market price data. Outcomes included lifetime reductions in tooth loss, total costs, quality-adjusted life years (QALYs) gained and incremental cost-effectiveness ratios (ICERs) compared with current practice. Cost-effectiveness was assessed against a willingness-to-pay threshold of $12,720.20 per QALY (one time GDP per capita), with threshold sensitivity analyses using one to three times GDP per capita.Results Brushing teeth twice daily was the only cost-saving intervention, reducing periodontitis by 50.2% relative to status quo, dental caries by 6.5% and tooth loss by 24.8%, while generating $718.82 billion in medical cost savings. The intervention yielded 148.27 million QALYs nationally (98.10 million in urban areas and 50.17 million in rural areas), with a greater reduction in periodontitis observed in rural regions (1.38 percentage points higher than urban). In contrast, dental flossing and mouthwash use increased costs and were not cost-effective at the one time GDP-per-capita threshold (ICERs of $49,677.58/QALY and $84,283.51/QALY, respectively). Findings were robust in threshold sensitivity analyses using one to three times GDP per capita.Conclusion Promoting twice-daily toothbrushing is cost-saving and improves oral health outcomes in China, particularly by reducing periodontitis and tooth loss, with larger periodontitis reductions in rural areas. These findings support the integration of basic oral hygiene practices into national health promotion and disease prevention strategies.