Background <p>Asthma is a serious global health issue, and obesity is a crucial risk factor for asthma. A Body Shape Index (ABSI) is an innovative metric for evaluating obesity. This study aimed to investigate the association between ABSI and asthma prevalence among middle-aged and older adults in China.</p> Methods <p>This cross-sectional study retrieved data from the third wave of the China Health and Retirement Longitudinal Study (CHARLS). We investigated the relationship between ABSI and asthma prevalence using multivariable logistic regressions, stratified analyses, and restricted cubic splines. Sensitivity analyses were conducted to ensure the robustness of the results.</p> Results <p>This study included 6,138 participants aged 45 years or older, among whom 365 were identified with asthma. A statistically significant association between ABSI Z-score and asthma was observed (adjusted odds ratio (OR) = 1.23, 95% CI: 1.06–1.42). A non-linear dose-response association was identified, with an inflection point at an ABSI Z-score of -0.523. For individuals with an ABSI Z-score ≥ -0.523, each unit increment in ABSI Z-score was associated with a 71% higher odds of asthma prevalence (OR = 1.71, 95% CI: 1.41–2.08). Significant interactions were observed in subgroups stratified by gender, smoking status, and marital status.</p> Conclusions <p>This study found a statistically significant association between ABSI and asthma prevalence in middle-aged and older Chinese. The data indicated that higher ABSI values were associated with increased asthma prevalence. Further prospective studies are warranted to elucidate temporal relationships and underlying causal mechanisms.</p> Clinical trial number <p>Not applicable.</p>