  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,
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.
