Abstract: A method for judging charge coupled device(CCD) imaging sensor output image signal quality by square wave contrast transfer function(CTF) is presented.The formulas for calculating the square wave CTF of CCD imaging sensor are given.This paper centers on using square wave CTF to analyze the spatial frequency response of ideal CCD systems.It is verified that the results from experiment are closed to the theory values.The research results will allow more accurate design of systems to discrete devices such as CCD.