  In this work, we simulate a set of realizations of local volume dark matter
subhalo population based on the distributions and relations derived from Via
Lactea II N-body simulation. We calculate the J-factors of these subhalos, and
find that the low mass subhalos contribute a lot to the total J-factors.
Combining with 91 months of the Fermi LAT observation, we constrain on the
cross section of dark matter annihilating directly to two gamma rays. This is
the first work combining numerical simulation results and Fermi LAT
observations to constrain dark matter cross section to gamma-ray line with
subhalo population. Though the constraints derived from subhalo population are
weaker than those from Fermi LAT observation of the Galactic center, they are
supports of and complementary to these other results.
