Acute coronary syndrome (ACS) is a special group of syndromes in coronary heart disease, mainly including unstable angina (UA), acute non-ST-segment elevation myocardial infarction (NSTEMI), and acute ST-segment elevation myocardial infarction (STEMI). They have an acute onset and have a higher morbidity and mortality rate compared to stable coronary artery disease, and if they occur, they require a high level of vigilance and attention from the patient to seek timely medical attention and treatment. The core of acute coronary syndrome is “instability”, and the main pathogenesis is the instability of atheromatous plaques in the coronary arteries, which rupture or erosion leading to intracoronary thrombosis. Unstable angina/acute non-ST-segment elevation myocardial infarction is caused by rupture or erosion of atherosclerotic plaque with varying degrees of surface thrombosis, vasospasm and distal vessel embolism, either of which results in a reduction in myocardial blood supply and imbalance in supply and demand. Unstable angina and acute non-ST-segment elevation myocardial infarction have similar etiology and clinical manifestations (anginal symptoms such as chest pain and tightness), and both may have ST-segment depression and or inversion or even no change on the ECG. However, the severity of myocardial ischemia and hypoxia differs, with the latter being more severe and persistent and leading to myocardial necrosis, at which time myocardial markers are checked for elevated cardiac enzymes (of which troponin I is the most specific). If thrombosis occurs on top of the ruptured or eroded plaque, resulting in continuous and complete occlusion of the coronary vessels, the ischemia and necrosis of the myocardium become more serious, and the patient’s chest pain and tightness become more serious, accompanied by a sense of dying, nausea, vomiting and other accompanying symptoms, which cannot be relieved by nitroglycerin; there will be ST-segment elevation on the ECG, accompanied by elevation of troponin I. In this case, we call it acute ST-segment elevation myocardial infarction (STEMI). Clinical symptoms, ECG, and myocardial enzymes play an important role in the diagnosis of acute coronary syndrome and in the differential diagnosis of unstable angina, acute ST-segment elevation myocardial infarction, and acute non-ST-segment elevation myocardial infarction. Acute coronary syndrome is a group of syndromes with acute onset, mainly including unstable angina pectoris, acute ST-segment elevation myocardial infarction, and acute non-ST-segment elevation myocardial infarction. If you have repeated symptoms of chest pain and chest tightness, you should seek medical attention promptly.