Thoracoscopically-assisted anterior spinal instrumentation is being used widely to treat adolescent idiopathic scoliosis (AIS). Recent studies have showed that screws placed thoracoscopically could counter the aorta or entrance into the spinal canal. There are a few studies defining the anatomic landmarks to identify the relationship between the aorta and the thoracic vertebral body using quantitative measurement for the sake of safe placement of thoracoscopic vertebral screw in anterior correction for AIS. The CT scanning from T4 to T12 in 64 control subjects and 30 AIS patients from mainland China were analyzed manually. Parameters to be measured included the angle for safety screw placement (alpha), the angle of the aorta relative to the vertebral body (beta), the distance from the line between the left and the right rib heads to the anterior wall of the vertebral canal (a), the distance from the left rib head to posterior wall of the aorta (b), the vertebral body transverse diameter (c) and vertebral rotation (gamma). No significant differences were found between the groups with respect to age or sex. Compared with the control group, alpha angle from T7 to T10, beta angle from T5 to T10 and b value at T9, T10 were significantly lower in the scoliotic group. The a value was significantly lower in the scoliotic group. The c value showed no significant difference between the two groups. In conclusion, to place the thoracoscopic vertebral screw safely, at the cephalad thoracic spine (T4-T6), the maximum ventral excursion angle should decrease gradually from 20 degrees to 5 degrees , the entry-point of the screw should be close to the rib head. For apical vertebrae (T7-T9), the maximum ventral excursion angle increased gradually from 5 degrees to 12 degrees. At the caudal thoracic spine (T10-T12), the maximum ventral excursion angle increased, the entry-point should shift 3 approximately 5 mm ventrally.