Objective To evaluate the correlations between cardiomyocyte injury markers and cardiac function, infarct volume and reperfusion injury in patients with ST segment elevation myocardial infarction (STEMI). Methods A total of 53 patients with acute STEMI were enrolled from Zhongshan Hospital of Fudan University from Sep. 2022 to Dec. 2023. Cardiac magnetic resonance (CMR) was done 4 to 7 d after percutaneous coronary intervention (PCI). The patients were divided into intramyocardial hemorrhage (IMH) group (33 cases) and non-IMH group (19 cases). The differences and correlations of CMR parameters and cardiomyocyte injury markers were analyzed between the 2 groups at different time points. The diagnostic value of cardiomyocyte injury markers at different time points for IMH was evaluated by receiver operating characteristic curve. Results Compared with the non-IMH group, the left ventricular ejection fraction of the IMH group was significantly decreased (［47.25±8.50］% vs ［55.08±9.01］%, P＝0.003), and the infarct volume percentage was significantly higher (［36.82±13.33］% vs ［19.73±10.90］%, P＜0.001). There was a negative correlation between IMH volume percentage and left ventricular ejection fraction in the IMH patients (r_s＝－0.640, P＜0.001). Cardiac troponin T after PCI immediately had a good effect for diagnosing IMH (area under curve was 0.890, P＜0.001), and it was negatively correlated with left ventricular ejection fraction (r_s＝－0.453, P＜0.01) and positively correlated with infarct volume percentage and IMH volume percentage (r_s＝0.540, P＜0.01; r_s＝0.773, P＜0.01). Conclusion IMH can lead to more severe myocardial infarction and has short-term adverse effects on left ventricular systolic function. Cardiomyocyte injury markers may provide a convenient way to detect IMH in STEMI patients after PCI. Higher cardiomyocyte injury markers indicate worse cardiac function, larger infarct volume and more severe reperfusion injury.