  The Five-Hundred-Meter Aperture Spherical Radio Telescope(FAST) is the
largest single-dish radio telescope in the world. In this paper, we make
forecast on the FAST HI large scale structure survey by mock observations. We
consider a drift scan survey with the L-band 19 beam receiver, which may be
commensal with the pulsar search and Galactic HI survey. We also consider
surveys at lower frequency, either using the current single feed wide band
receiver, or a future multi-beam phased array feed (PAF) in the UHF band. We
estimate the number density of detected HI galaxies and the measurement error
in positions, the precision of the surveys are evaluated using both Fisher
matrix and simulated observations. The measurement error in the HI galaxy power
spectrum is estimated, and we find that the error is relatively large even at
moderate redshifts, as the number of positively detected galaxies drops
drastically with increasing redshift. However, good cosmological measurement
could be obtained with the intensity mapping technique where the large scale HI
distribution is measured without resolving individual galaxies. The figure of
merit (FoM) for the dark energy equation of state with different observation
times are estimated, we find that with the existing L-band multi-beam receiver,
a good measurement of low redshift large scale structure can be obtained, which
complements the existing optical surveys. With a PAF in the UHF band, the
constraint can be much stronger, reaching the level of a dark energy task force
(DETF) stage IV experiment.
