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The acute myocardial infarction (AMI) outcomes have been extensively linked with ambient particulate matter (PM). However, whether a smaller particle has greater impact and the consequent attributable burden associated with PM of different sizes remain unclear. We conducted a multi-province cross-sectional study among AMI patients using the inpatient discharge datasets from four Chinese provinces (Shanxi, Sichuan, Guangxi, and Guangdong) from 2014 to 2019. Ambient PM exposure for each patient was assessed using the ChinaHighAirPollutants dataset. We employed the mixed-effects logistic regression models to evaluate the association of PM of different sizes (PM 1 , PM 2.5 , PM 10) on in-hospital case fatality. The potential reducible fractions in in-hospital case fatality were estimated through counterfactual analyses. Of 177,749 participants, 125,501 (70.6 %) were male and the in-hospital case fatality rate was 4.9%. For short-term (7-day average) exposure, the odds ratios (ORs) for PM 1 , PM 2.5 , and PM 10 (per 10 µg/m3) were 1.052 (95 % confidence interval [CI], 1.032–1.071), 1.026 (95 % CI, 1.014–1.037), and 1.016 (95% CI, 1.008–1.024), respectively. The estimated ORs for long-term exposure (annual average) were 1.303 (95 % CI, 1.252–1.356) for PM 1 , 1.209 (95 % CI, 1.178–1.241) for PM 2.5 , 1.157 (95 % CI, 1.134–1.181) for PM 10. Short-term exposure to PM 1 showed the highest potential reducible fraction (8.5 %, 95 % CI, 5.0–11.7 %), followed by PM 2.5 and PM 10 , while the greatest potential reducible fraction of long-term exposure was observed in PM 10 (30.9 %, 95 % CI, 27.2–34.4%), followed by PM 2.5 and PM 1. In summary, PM with smaller size had a more pronounced impact on in-hospital AMI case fatality, with PM 1 exhibiting greater effects than PM 2.5 and PM 10. Substantial health benefits for AMI patients could be achieved by mitigating ambient PM exposure. • Evidence on the association between AMI case fatality and small particles is scarce. • The effect of PM 1 on in-hospital AMI case fatality was stronger than PM 2.5 and PM 10. • The largest population reducible fraction was observed in long-term PM 10 exposure. [ABSTRACT FROM AUTHOR]