Objective To explore the prognostic value of the coronary angiography-derived index of microcirculatory resistance (caIMR) for major adverse cardiovascular events (MACE) in patients with acute ST-segment elevation myocardial infarction (STEMI) after primary percutaneous coronary intervention (PCI). Methods Between September 2019 and March 2022, 541 patients diagnosed with STEMI at the Affiliated Hospital of Xuzhou Medical University were enrolled. The caIMR was calculated using the FlashAngio system (Suzhou Rainmed Medical Technology Co., Ltd.). The patients were divided into MACE and non-MACE groups according to the occurrence of MACE during hospitalization or follow-up, with MACE defined as all-cause mortality, heart failure readmission, and unplanned revascularization. COX regression analysis, receiver operating characteristic (ROC) curves, and Kaplan-Meier survival curves were used to evaluate the prognostic value of caIMR for STEMI patients after primary PCI. Results During the 1-year follow-up, 61 patients (11.28%) experienced MACE. The patients in the MACE group had higher caIMR values than those in the non-MACE group. Multivariate COX analysis showed that caIMR was an independent risk factor for MACE. ROC curve analysis showed that caIMR predicted MACE with an area under the curve of 0.688, and the optimal cutoff value was 25.3 U. caIMR significantly increased the discriminant and reclassification indexes when added to a model with clinical risk factors. The patients were further divided into a low caIMR group (caIMR<25 U, n=377) and a high caIMR group (caIMR≥25 U, n=164). Kaplan-Meier curve showed that patients with caIMR≥25 U had a worse prognosis. Conclusions caIMR is an independent risk factor for poor prognosis after PCI in patients with STEMI, and patients with caIMR≥25 U had a worse prognosis.