Abstract: Precise star-centroid determination of digital star image screened with CCD is important to estimate the precise astronomical location of observing station in the surveying of vertical deflection based on GPS and CCD with the astro-geodetic method. The sub-pixel center of CCD star image should be positioned to improve the precision of station astronomical latitude and longitude with the digital zenith camera. A program is developed with MATLAB to read CCD image in FITS format in the paper. Based on the analysis of CCD star image, the two-dimensional moment method is improved to process the real CCD star image. The threshold value selected in the moment method in common use can affect the positioning precisions of star images with different brightness. So a iterative method to calculate the optimal threshold is put forward. Comparing this method with the two-dimensional modified moment method, we find that the improved two-dimensional moment method is of more efficiency and reliability. We get more precise positioning results for faint stars with the improved method and the positioning precision for faint stars is up to 0.1-0.15 pixel.