  Intensity mapping of 21-cm line by several radio telescope experiments will
probe the large-scale structure of the Universe in the post-reionization epoch.
It requires a theoretical framework of neutral hydrogen (HI) clustering, such
as modelling of HI power spectrum for Baryon Acoustic Oscillations (BAO)
analysis. We propose a new method for reconstructing the HI map from dark
matter distribution using N-body simulations. Several studies attempt to
compute the HI power spectrum with N-body simulations by pasting HI gas at the
dark matter halo centre, assuming the relation between the halo and HI masses.
On the other hand, the method proposed in this paper reproduces the HI power
spectrum from simulated dark matter distribution truncated at specific scales
from the halo centre. With this method, the slope of HI power spectrum is
reproduced well at the BAO scales, $k<1 h/{\rm Mpc}$. Furthermore, we find the
fluctuation of spin temperature, which is often ignored at the
post-reionization epoch, alters the power spectrum of brightness temperature by
at most 8% in the power spectrum. Finally, we discuss how our method works by
comparing the density profiles of HI and dark matter around the dark matter
halos.
