Abstract: Artifacts will be introduced into the reconstruction for fan-beam CT system if the central X-ray is not perpendicular to the detector array, and the image quality will degrade consequently. In this study, the reasons for the formation of such artifacts are analyzed based on the classic theory of filtered back projection algorithm, and a novel method is proposed to calculate the geometric parameters necessary based on sinogram features. The image quality can be improved when we put these parameters into the revised algorithm. The newly proposed method requires no correction phantom, thus avoiding twice scan. Computer simulation and experimental results demonstrate that the method is easy, fast, and effective for practical utilization.