Aim To compare multiple ECG testing measurements with coronary angiography and study the diagnostic ability of ST segment horizon tal elongation on the coronary stenosis severity. Methods CAG and ECG were performed in 334 cases respectively. Results Sensitivity of diagnosing CAG of ST segment horizontal elongtion (≥0.12 s) is superior to ST segment depression and ischemic T waves. In mild, moderate and severe CAD cases, the comparison of the sensitivity is as follows:51%vs41%,38%; 75%vs 52%,56%(p<0.01);100% vs 96%, 100%(p>0.05). Moreover,comparison between ST segment horizontal elongation (≥0.12 s) and CAG demonstrated: When the stenosis is less than 90% in both single and multiple coronary arteries disease, the positive rate is 47%,68%; When the stenosis is more than 90% in both single and multiple coronary arteries disease, the positive rate is 73%,100%.(p<0.01) The difference is significant between them. Conclusion The prolongation of ST segment may not only serve as one of marks for the diagnosis of early CAD because of its higher sensitivity but also be used for the anglysis of the severity of coronary stenosis because of its important values to detect severe CAD.