A method for reconstructing a CT scan image, comprising: performing a first scan to acquire a first projection data, wherein the first scan is a scan that a subject is covered by a low dose X-ray beam in an axial plane； performing a second scan to acquire a second projection data, wherein the second scan is a scan that a target portion of the subject is covered by a high dose X-ray beam in the axial plane； determining a data filling point in the second projection data, wherein the data filling point is located in a truncated region； and filling the data filling point in the second projection data with a data from the first projection data which corresponds to the data filling point； and reconstructing a second scan image based on the filled second projection data.
