Acute coronary syndrome (ACS) is one of the main causes of death. Worldwide, tens of millions of patients are hospitalized for coronary heart disease and ACS every year. ACS may show acute myocardial infarction, unstable angina pectoris, and even induce early arrhythmia, leading to sudden death. Sudden cardiac death (SCD) has a strong correlation with ACS. Data from clinical and autopsy studies and death certificates show that 62-85% of patients with out of hospital SCD have a history of ACS, 10% have other cardiac structural abnormalities, and 5% have no cardiac structural abnormalities. An SCD surveillance study from Ireland concluded that most cases occurred in families, and the successful recovery of SCD was mainly related to ventricular fibrillation with arrhythmia. At present, there are few reports on the clinical and prognosis of ACS in China, and there is no guideline or consensus on the prevention and treatment of ACS patients. Known domestic research results show that the proportion of male, overweight / obese, smokers in young ACS patients is higher than that in the elderly group, while the proportion of patients with hypertension, diabetes and cerebrovascular diseases is less than that of the elderly group. The levels of TC, TG, LDL-C and UA and the proportion of low HDL-C in young ACS patients were higher than those in elderly patients; The diagnosis of STEMI was the highest in the young ACS group, while the diagnosis of unstable angina pectoris was the most common in the elderly ACS group. The clinical manifestations of ACS vary greatly. 3% \ 5% of patients who exclude ACS only through myocardial markers, clinical blood transfusion and ECG still have myocardial infarction (MI). For the emergency department, early prediction of the risk of SCD in ACS patients and timely and accurate screening of high-risk patients are very important.