  The 21-cm intensity mapping (IM) of neutral hydrogen (HI) is a promising tool
to probe the large-scale structures. Sky maps of 21-cm intensities can be
highly contaminated by different foregrounds, such as Galactic synchrotron
radiation, free-free emission, extragalactic point sources, and atmospheric
noise. We here present a model of foreground components and a method of
removal, especially to quantify the potential of Five-hundred-meter Aperture
Spherical radio Telescope (FAST) for measuring HI IM. We consider 1-year
observational time with the survey area of $20,000\,{\rm deg}^{2}$ to capture
significant variations of the foregrounds across both the sky position and
angular scales relative to the HI signal. We first simulate the observational
sky and then employ the Principal Component Analysis (PCA) foreground
separation technique. We show that by including different foregrounds, thermal
and $1/f$ noises, the value of the standard deviation between reconstructed
21-cm IM map and the input pure 21-cm signal is $\Delta T = 0.034\,{\rm mK}$,
which is well under control. The eigenmode-based analysis shows that the
underlying HI eigenmode is just less than $1$ per cent level of the total sky
components. By subtracting the PCA cleaned foreground+noise map from the total
map, we show that PCA method can recover HI power spectra for FAST with high
accuracy.
