In acquisition, pointing and tracking (APT) system of optical communications, the resolution of detector is very important for whole system. In order to meet the specification of the communications, improve the performance of APT ulteriorly, a new method is put forward in this paper, it is multiple-sampling for detecting data. According to this method, two results of sampling are added together and disposed by procedure, this can make resolution of charge coupled device (CCD) attain sub-pixel. From the simulation of this method, the resolutions of CCD get to 1/2, 1/4 pixel or more. At the same time, this method can also improve the signal-noise-ratio (SNR) of data. So, the method has been proved to be effective to improve the resolution to sub-pixel for CCD in APT system.