In the photographic process with Charge Coupled Device (CCD), the image sampling of CCD is important. Due to the previous incorrect mathematical descriptions of the sampling, a correct mathematical equation is given by analyzing the CCD image acquisition process. Comparing the previous and the given one, shortcomings of the previous are pointed out. Then, a detailed spectrum analysis of the image sampling is carried out which illustrates the influence of various parameters of CCD on image acquisition. The mathematical analysis can optimize the choice of the parameters of CCD in practical applications.