  The intensity mapping of Ly-alpha emission during the epoch of reionization
(EoR) will be contaminated by foreground emission lines from lower redshifts.
We calculate the mean intensity and power spectrum of Ly-alpha emission at z~7,
and estimate the uncertainties according to the relevant astrophysical
processes. We find that the low-redshift emission lines from 6563 A H-alpha,
5007 A [OIII] and 3727 A [OII] will be the strong contaminants on the observed
Ly-alpha power spectrum. We make use of both the star formation rate (SFR) and
luminosity functions (LF) to estimate the mean intensity and power spectra of
the three foreground lines at z~0.5 for H-alpha, z~0.9 for [OIII] and z~1.6 for
[OII], as they will contaminate the Ly-alpha emission at z~7. The [OII] line is
found to be the strongest. We analyze the masking of the bright survey pixels
with a foreground line above some line intensity threshold as a way to reduce
the contamination in the intensity mapping survey. We find that the foreground
contamination can be neglected if we remove the pixels with fluxes above
1.4x10^-20 W/m^2.
