Disclosed is a method for positioning a target in a three-dimensional CT image and a CT system for security inspection. The method includes: displaying a three-dimensional CT image; receiving a selection by a user of at least one area of the three-dimensional CT image at a viewing angle; generating at least one set of three-dimensional objects in a depth direction based on the selection; and determining a target object from the set. With the above technical solutions, the user may be facilitated in marking a suspected object in a CT image in a quick manner.