This randomized, multicenter, prospective trial will enroll 6,294 participants in approximately 80 centers. STEMI patients with onset time within 12 hours and scheduled for primary PCI will be randomly assigned to two groups in a 1:1 ratio. Patients meeting the criteria were randomly assigned. It is recommended that loading doses of dual antiplatelet therapy be administered immediately after electrocardiogram diagnosis. The experimental group was required to be given an intravenous injection of 100 U/kg of UFH within 10 minutes after randomization, and this should be completed at least before delivery to the catheter lab. The control group was recommended to be given 100 U/kg of UFH through the arterial sheath, but the actual intraoperative dosage was determined by the interventional cardiologist. No patient is allowed to use low-molecular-weight heparin, bivalirudin, glycoprotein IIb/IIIa inhibitors or other antithrombotic drugs before coronary angiography. The planned enrollment period for this study is 24 months. The scheduled follow-up visits will occur at 30 days (±7 days), 3 months (±14 days), and if conditions permit, at 6 months (±30 days) and 1 year (±30 days) after randomization. The purpose of this study is to evaluate the composite endpoint of all-cause death, recurrent myocardial infarction, urgent coronary revascularization, stent thrombosis, or stroke within 30 days after randomization.

Inclusion Criteria:

1. Age ≥18 years old;

2. STEMI within 12 hours of onset Newly developed adjacent two or more leads with ST segment elevation of ≥1mm (lead V2-V3 elevation of ≥2mm).

   Newly developed left bundle branch block (LBBB). In the right bundle-branch block (RBBB), the ST segment of leads V1-V3 is elevated or Q waves occur.

3. Primary PCI was planned if the time from first medical contact to balloon dilatation was expected to be less than 120 minutes.

Exclusion Criteria:

1. Thrombolytic therapy recipients;
2. Currently taking oral anticoagulant drugs or having been treated with heparin, low molecular weight heparin, suldaparinol sodium, bivalirudin or IIb/IIIa receptor antagonists within 48 hours before randomization;
3. Patients undergoing cardiopulmonary resuscitation;
4. Patients with cardiogenic shock;
5. Combined mechanical complications (rupture of the free wall of the heart, perforation of the ventricular septum, insufficiency or rupture of the papillary muscle leading to severe mitral regurgitation);
6. History of intracranial parenchymal aneurysm, intracranial arteriovenous malformation, intracranial hemorrhage, ischemic stroke or transient ischemic attack within the last 6 months, and active hemorrhage within the last 2 weeks;
7. Major surgery within one month;
8. Previous history of CABG;
9. Patients with a history of heparin-induced thrombocytopenia or those known to be allergic to anticoagulant or antiplatelet drugs;
10. Combined with other serious diseases and life expectancy less than 12 months;
11. Pregnant or lactating women;
12. Currently participating in clinical research on other drugs or devices;
13. Refuse to sign the informed consent form;
14. The researcher judged that it was not suitable to participate in this study.