Abstract: A cardiac diagnostic method includes the following steps: providing an electrocardiogram (ECG) signal; obtaining a body surface potential vector according to the ECG signal; calculating the body surface potential vector and a pre-created mapping tensor to obtain a cardiac potential vector, wherein the mapping tensor is obtained according to a plurality of abnormal positions and a plurality of degree of abnormalities generated by a reference heart; and classifying the abnormal positions and the degree of abnormalities of a detected heart corresponding to the ECG signal according to the cardiac potential vector. In addition, the present disclosure further provides a non-transitory computer-readable medium and system that can operate the above-mentioned cardiac diagnostic method.