Objective To investigate the correlation between preoperative free triiodothyronine (FT3) to free thyroxine (FT4) (FT3/FT4) ratio and post-contrast acute kidney injury (PC-AKI) in the patients with acute coronary syndrome (ACS). Methods A retrospective analysis was performed of the clinical data of 10101 ACS patients undergoing percutaneous coronary intervention (PCI) in the General Hospital of Northern Theater Command from January 2016 to December 2017. The FT3/FT4 ratio was calculated based on the laboratory tests, and the patients were divided into three groups according to the FT3/FT4 ratio:low FT3/FT4 group (FT3/FT4＜0.233, n=3152),intermediate FT3/FT4 group(0.233≤FT3/FT4＜0.304, n=3636), and high FT3/FT4 group (FT3/FT4≥0.304, n=3313). The endpoint of the study was defined as the incidence of PC-AKI in the ACS patients undergoing PCI. SAS 9.4 was used for statistical analysis. Depending on data type, data comparison among three groups were performed by analysis of variance, Kruskal-Wallis test orχ²test. The correlation between preoperative FT3/FT4 ratio and the occurrence of PC-AKI was analyzed using restricted cubic spline. Results There were statistically significant differences among the three groups in age, proportion of men, proportion of hypertension, proportion of diabetes mellitus, history of previous PCI, smoking, left ventricular ejection fraction, hemoglobin level, FT3 and FT4 levels, proportion of radial artery approach, target vessel at the anterior descending branch, and intraoperative use of contrast agent (P<0.05 for all). The incidence of PC-AKI was 2.95% (93/3152), 1.27% (46/3636), and 1.54% (51/3313) in the low, intermediate, and high FT3/FT4 group, respectively, and the differences were statistically significant (P<0.001). Preoperative FT3/FT4 ratio and the incidence of PC-AKI in ACS patients showed a U-shaped relationship. Using the FT3/FT4 ratio of 0.305 as the reference value, the risk of PC-AKI tended to increase with either lower or higher FT3/FT4 ratios. Conclusion FT3/FT4 ratio is closely related to the occurrence of PC-AKI in ACS patients, and the risk of PC-AKI increases with either lower or higher FT3/FT4 ratios.