Objective: Non-ST-elevation myocardial infarction (NSTEMI) complicated by acute heart failure (AHF) frequently leads to unfavorable outcomes. The inflammatory response and immune dysregulation play a crucial role in the development and progression of this disease. This study aimed to explore the predictive value of the systemic immune inflammation index (SII) in the early development of AHF in NSTEMI patients. Methods: A retrospective study was carried out to collect the clinical data of NSTEMI patients admitted to the cardiovascular department of the First Affiliated Hospital with Nanjing Medical University (Jiangsu Provincial People's Hospital) from September 2023 to September 2024. Spearman correlation analysis was employed to investigate the correlation between the admission SII and the peak value of N-Terminal Pro-Brain Natriuretic Peptide (NT-proBNP) within one week after admission. Based on the receiver operating characteristic curve (ROC) and the Restricted Cubic Spline based on logistic regression (RCS) analysis, the specific relationship between SII and the risk of AHF in NSTEMI patients was explored. Results: A total of 202 NSTEMI patients were included, among whom 104 (51.5%) developed AHF within one week of admission. The level of SII in patients with AHF was significantly higher than that in patients without AHF [1390.55 (939.45, 2459.93) vs 667.15 (431.58, 1140.25), P < 0.001]. The area under the ROC curve was 0.745 (95%CI: 0.6777-0.812, P < 0.001). Spearman correlation analysis indicated that there was a linear correlation between the admission SII and the peak value of NT-proBNP within one week of admission, and the correlation coefficient was 0.317 (P < 0.001). The RCS curve demonstrated a linear positive correlation between the risk of early AHF in NSTEMI patients and SII. Conclusion: The SII exhibits predictive value for the early onset of AHF in NSTEMI patients, with the risk of early AHF increasing as SII levels rise.