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The cross-correlation of optical galaxies with the neutral hydrogen (HI)radiation intensity can enhance the signal-to-noise ratio (SNR) of the HIintensity measurement. In this paper, we investigate the cross-correlation ofthe galaxy samples obtained by the spectroscopic survey of the China SpaceStation Telescope (CSST) with the HI Intensity mapping (IM) survey of theFive-hundred-meter Aperture Spherical Telescope (FAST). Using the IllusitrisTNGsimulation result at redshift $0.2 \sim 0.3$, we generate mock data of the CSSTsurvey and a FAST L-band drift scan survey. The CSST spectroscopic survey canyield a sample of galaxies with a high comoving number density of $10^{-2}(\unit{Mpc}/h)^{-3}$ at $z \sim 0.3$. We cross-correlate the foreground-removedradio intensity with the CSST galaxies, including both the whole sample, andred and blue galaxy sub-samples separately. We find that in all cases the HIand optical galaxies are well correlated. The total HI abundance can bemeasured with a high precision from this correlation. A relative error of $\sim0.6\%$ for $\Omega_{\rm HI}$ could be achieved at $z\sim 0.3$ for anoverlapping survey area of $10000 \unit{deg}^2$.

Forecasting the Cross-Correlation of the CSST galaxy survey with the FAST HI Intensity Map