Objective To investigate the clinical value by reducing contrast material injection time and optimizing the dosage on the quality of image of coronary CT in 256-row spiral CT scanner. Methods A total of 100 consecutive patients who had done coronary CTA were divided into A group (injection time 10s,50 cases)and B group(injection time 8s,50m cases) randomly, which use the same contrast agent and the injection velocity. We place the area of interest in start section of the ascending aortic root cavity and main coronary artery(right coronary artery, left coronary artery, left anterior descending branch of coronary artery, left circumflex artery) and adjacent tissue, measuring the CT value and standard deviation (SD). Compared the CT value, the noise ratio(CNR) of the aortic and coronary artery by using paired T-test. Subjectively compared the coronary CT image quality (CCTA) between two groups through using 5 points methods(5 is the best,1 is the worst) and wilcoxon symbols rank test. Contrast dose and radiation dose were measured and calculated. Results There are no statistically significance of ED between the two groups (P=0.50), the ED of group A and group B was (46.16±6.90)mL, (35.74±4.81) mL respectively. The aortic root,right coronary artery, left main coronary artery, the left anterior descending branch of coronary artery, the start section of left circumflex artery of CT value and vascular contrast to noise ratio (CNR) in two groups has no difference (P>0.05). Conclusion In 256-slice spiral CT coronary angiography, the choice of 8s contrast agent injection time can effectively reduce the amount of contrast agent, but the image quality is not affected. .