This study aims to investigate the imaging quality and radiation dose of prospective ECG-triggered 256 multi-slice computer tomography (MSCT) in accessing the coronary artery (CA) in children. Coronary arteries of 149 children were evaluated using prospective ECG-triggered 256 MSCT with the same system settings. A four-point scoring system was applied to study the capability of MSCT in detecting CA in these patients. Signal, noise and contrast-to-noise ratios (CNR) were analyzed to investigate the association of image quality with age. Then, volumetric CT dose index (CTDI_vol), dose-length product (DLP), and effective dose (ED) were utilized to study the association of radiation dose with age. The detection rate for original, proximal, middle, distal and the 11 segments of CA was 100, 97, 92, 81 and 92%, respectively; and there was no influence on age during the detection (all, P < 0.05). A negative correlation was found between ED and age (r = −0.664, P < 0.001). Significantly larger EDs were found in younger patients (age: <3 years; 1.2 ± 0.5 and 0.6 ± 0.2 mSv; P < 0.001), while higher DLPs were found in elder patients, although no correlation was found between ages versus DLP (r = 0.092, P = 0.262). Prospective ECG-triggered 256 MSCT has considerable performance for the evaluation of CA in children. However, great caution is needed for children under the age of three in the selection of this examination. Furthermore, the tube current could be further reduced for the examination of children ≥8 years.