Abstract: The faithfulness of a CT image depends on a number of system-level performance factors,with one of the most important being spatial resolution.Spatial resolution refers to the ability of a CT system to resolve small details or locate small feature with respect to some reference point.Spatial resolution is generally quantified in terms of the smallest separation at which two points can be distinguished as separate entities.The limiting value of the spatial resolution is determined by the design and construction of the system.The limit spatial resolution is determined by the cut-off frequency(COF),which is approximately 1/BW.The optimal magnification is derived by the relationship equation in this paper.This discovery of this principle will be helpful to getting the best image for the testing personnel in his work.At the same time it will also provide a theoretical basis for the design of the CT system.