Objective To explore the relationship between activated partial thromboplastin time (aPTT) and intracoronary thrombus burden in patients with acute myocardial infarction. Methods From January 2011 to December 2013, a total of 424 patients with acute ST-segment elevation myocardial infarction were enrolled, who were admitted to the Emergency Department of Beijing Friendship Hospital affiliated to Capital Medical University and received primary percutaneous coronary intervention (PCI) within 6 hours. According to the findings of coronary artery angiography and the coronary intervention, they were divided into the high thrombus burden (HTB) group (n=199) and the low thrombus burden (LTB) group (n=225). Baseline hematologic indices and thrombus burden were measured on the visit at the Emergency Department. SPSS statistics 19.0 was used to process the data and multivariate logistic regression analysis was employed for comparison between groups. Results Compared with the LHB group, the HTB group had lower average age[(59.4±11.7) vs(61.9±11.8)years], aPTT[(24.9±3.2) vs(26.6±4.0)s], PT [(11.3±0.8) vs(11.5±1.0)s]and LCX proportion[9.5%(19/199) vs 18.0%(40/225)], but higher male proportion[84.3%(168/199) vs 74.3%(167/225)] , white blood cell count[(10.0±3.1)×10⁹ vs(9.3±3.1)×10⁹/L] and RCA proportion [45.2%(90/199) vs 29.8%(68/225)], the differences being statistically significant (all P<0.05). aPTT in the HTB group was significantly lower than that of the LTB group. There was no statistically significant difference between the two groups in smoking status, medication, previous hypertension and diabetes, liver function, kidney function, electrolytes, PTA, AT-Ⅲ, FBG and other indicators (P>0.05). Multivariate logistic regression analysis showed aPTT (OR=1.175,95%CI 1.102-1.252) and RCA (OR=2.783,95%CI 1.409-5.497)as independent predictors for thrombus burden (P<0.01). The area under the receiver operating characteristic curve of aPTT was 0.660 (95%CI 0.608-0.711, P<0.001). The results suggest that the best cut-off value for aPTT excluding high thrombus load was 26.05 with a sensitivity of 53.5% and a specificity of 72.6%. Conclusion Shortened aPTT on admission is significantly related to angiographic thrombus burden in the patients with acute ST-segment elevation myocardial infarction.