  The Chinese Space Station Telescope (CSST) slitless spectroscopic survey will
observe objects to a limiting magnitude of ~ 23 mag (5$\sigma$, point sources)
in U, V, and I over 17500 deg$^2$. The spectroscopic observations are expected
to be highly efficient and complete for mapping galaxies over 0 < z < 1 with
secure redshift measurements at spectral resolutions of R ~ 200, providing
unprecedented data sets for cosmological studies. To quantitatively examine the
survey potential, we develop a software tool, namely the CSST Emulator for
Slitless Spectroscopy (CESS), to quickly generate simulated 1D slitless spectra
with limited computing resources. We introduce the architecture of CESS and the
detailed process of creating simulated CSST slitless spectra. The extended
light distribution of a galaxy induces the self-broadening effect on the 1D
slitless spectrum. We quantify the effect using morphological parameters:
S\'ersic index, effective radius, position angle, and axis ratio. Moreover, we
also develop a module for CESS to estimate the overlap contamination rate for
CSST grating observations of galaxies in galaxy clusters. Applying CESS to the
high-resolution model spectra of a sample of ~ 140 million galaxies with m_z <
21 mag selected from the Dark Energy Spectroscopic Instrument LS DR9 catalogue,
we obtain the simulated CSST slitless spectra. We examine the dependence of
measurement errors on different types of galaxies due to instrumental and
observational effects and quantitatively investigate the redshift completeness
for different environments out to z ~ 1. Our results show that the CSST
spectroscopy is able to provide secure redshifts for about one-quarter of the
sample galaxies.
