  The Chinese Space Station Optical Survey (CSS-OS) is a mission to explore the
vast universe. This mission will equip a 2-meter space telescope to perform a
multi-band NUV-optical large area survey (over 40% of the sky) and deep survey
(~1% of the sky) for the cosmological and astronomical goals. Galaxy detection
is one of the most important methods to achieve scientific goals. In this
paper, we evaluate the galaxy number density for CSS-OS in i band (depth, i ~26
for large area survey and ~27 for the deep survey, point source, 5-sigma by the
method of image simulation. We also compare galaxies detected by CSS-OS with
that of LSST (i~27, point source, 5-sigma. In our simulation, the HUDF galaxy
catalogs are used to create mock images due to long enough integration time
which meets the completeness requirements of the galaxy analysis for CSS-OS and
LSST. The galaxy surface profile and spectrum are produced by the morphological
information, photometric redshift and SEDs from the catalogs. The instrumental
features and the environmental condition are also considered to produce the
mock galaxy images. The galaxies of CSS-OS and LSST are both extracted by
SExtractor from the mock i band image and matched with the original catalog.
Through the analysis of the extracted galaxies, we find that the effective
galaxy number count is ~13 arcmin^-2, ~40 arcmin^-2 and ~42 arcmin^-2 for
CSS-OS large area survey, CSS-OS deep survey and LSST, respectively. Moreover,
CSS-OS shows the advantage in small galaxy detection with high spatial
resolution, especially for the deep survey: about 20% of the galaxies detected
by CSS-OS deep survey are not detected by LSST, and they have a small effective
radius of re < 0.3".
