This is a double-blind, multicenter, randomized, placebo-controlled clinical trial. It is planned to enroll patients admitted with anterior ST-segment elevation myocardial infarction (STEMI) within 6h of symptom onset and undergo primary percutaneous coronary intervention (pPCI). Patients who meet the inclusion criteria and without exclusion criteria were randomized 1:1 into the dexmedetomidine (DEX) group or the placebo (saline) group after signing the informed consent. In the DEX group, intravenous injection of DEX was started immediately after enrollment, covering the entire PCI operation, and the administration was stopped at the end of the pPCI. The administration of saline was the same as those in the DEX group. The primary endpoint was the myocardial infarct size (MIS) as assessed by cardiac magnetic resonance imaging (CMR) at 5±2 days post-STEMI. Based on a superiority design and assuming an 20.0% relative infarct size reduction (from 26.0% to 20.8% with a SD of 13.0%), 250 patients are required to be enrolled, accounting for 20% drop-out (α= 0.05 and power= 80%).