Sudden cardiac death (SCD) caused by acute ischemic heart disease (IHD) is a major cause of sudden death worldwide. Circular RNAs (circRNAs) are abundant in the heart and play important roles in cardiovascular diseases, but the role of circRNAs as biomarkers in the forensic diagnosis of SCD caused by acute IHD remains poorly characterized. To investigate the potential of two heart-enriched circRNAs, circNFIX and circSLC8A1, we explored the expression of these two circRNAs in different kinds of commonly used IHD models, and further verified their expressions in forensic autopsy cases. The results from both the IHD rat and H9c2 cell models revealed that circSlc8a1 level was upregulated, while the circNfix level was elevated in the early stage of ischemia and subsequently downregulated. The time-dependent expression patterns of the two circRNAs suggested their potential as SCD biomarkers. In autopsy cases, the results showed that the expression of these two circRNAs in the myocardium with acute IHD-related SCDs corresponded to the observations in the ischemic models. Further analysis related to myocardial ischemia indicated that circSLC8A1 showed high sensitivity and specificity for myocardial infarction and was positively correlated with creatine kinase MB in pericardial fluid. Downregulated circNFIX level could indicate the ischemic myocardial damage, and it was negatively correlated with the coronary artery stenosis grade. The combination of circSLC8A1 and circNFIX had better performance to discriminate IHD-related SCDs. The results suggested that circSLC8A1 and circNFIX may be used as auxiliary diagnostic markers for SCD caused by acute IHD in forensic medicine.