Abstract: Objective To assess the effect of lymphocyte to C-reactive protein ratio (LCR) on new-onset atrial fibrillation (NOAF) in patients with ST-segment elevation myocardial infarction (STEMI) who underwent primary percutaneous coronary intervention (pPCI). Methods Patients who were diagnosed with STEMI and successfully underwent pPCI in the Affiliated Hospital of Xuzhou Medical University from June 2020 to December 2023 were selected and their clinical data were retrospectively analyzed. All patients received continuous ECG monitoring during hospitalization. According to the presence of NOAF, they were divided into two groups: a NOAF group and a non-NOAF group. Univariate and multivariate logistic regression analyses were conducted to screen out the influencing factors of NOAF. Results The incidence of NOAF during hospitalization was 7.2%(44/607). Univariate logistic regression analysis showed that age, left ventricular ejection fraction (LVEF), LCR, Killip grade ≥2, and the right coronary artery (RCA) were associated with the occurrence of NOAF (P<0.05). According to multivariate logistic regression analysis, age (OR=1.037, 95%CI: 1.009-1.067) and RCA (OR=3.118, 95%CI: 1.622-5.995) were the risk factors for NOAF, while LVEF (OR=0.935, 95%CI: 0.894-0.978) and LCR (OR=0.067, 95%CI: 0.009-0.473) were the protective factors for NOAF. Integration of LCR significantly improved the predictive ability of the model for NOAF (NRI=0.472, IDI=0.035, P<0.001). Conclusions Preoperative LCR level is an independent predictor of NOAF after pPCI in STEMI patients, which can well predict NOAF after pPCI in STEMI patients.