The invention discloses a CT ultralow-dose automatic three-dimensional positioning scanning method and a system. The method comprises the following steps: under the condition of conventional radiationdose, scanning to obtain a normal dose image, adding quantum noise and electronic noise into the normal dose image to generate an ultralow dose spiral scanning image, and performing AI low dose reconstruction to obtain a three-dimensional positioning image for CT scanning. According to the invention, under the conditions of not stopping the rotation of the rack and adopting similar radiation doselevels, the three-dimensional positioning image is directly obtained, the area needing to be scanned is automatically positioned, automatic scanning is carried out, the positioning accuracy and scanning efficiency are improved, meanwhile, the radiation dose of a patient can be reduced, and the requirements for the technical level, experience and the like of scanning operators are reduced.