Abstract: It is very difficult to modulate a real industrial CT system to satisfy the geometric alignment relations required by the conventional CT image reconstruction formula. A misalignment will result in geometric artifacts in the image reconstructed by the conventional formula, which may cause a misjudgment. In this paper, we study the impact of geometric parameters errors for fan-beam system on the reconstructed images of a thin wire. We derive the envelope function which describes geometric artifact configuration in the image. Based on the envelope function, we give a classification, the evolution regularity and the characteristics of geometric artifacts. Besides, we give a method to adjust the errors of geometric parameters.