  Recently, ultraluminous X-ray source (ULX) M82 X-2 has been identified to be
an accreting neutron star, which has a $P=1.37$ s spin period, and is spinning
up at a rate $\dot{P}=-2.0\times 10^{-10}~\rm s\,s^{-1}$. Interestingly, its
isotropic X-ray luminosity $L_{\rm iso}=1.8\times 10^{40}~\rm erg\,s^{-1}$
during outbursts is 100 times the Eddington limit for a $1.4~\rm M_{\odot}$
neutron star. In this Letter, based on the standard accretion model we attempt
to constrain the dipolar magnetic field of the pulsar in ULX M82 X-2. Our
calculations indicate that the accretion rate at the magnetospheric radius must
be super-Eddington during outbursts. To support such a super-Eddington
accretion, a relatively high multipole field ($\ga 10^{13}$ G) near the surface
of the accretor is invoked to produce an accreting gas column. However, our
constraint shows that the surface dipolar magnetic field of the pulsar should
be in the range of $1.0-3.5\times 10^{12}$ G. Therefore, our model supports
that the neutron star in ULX M82 X-2 could be a low magnetic field magnetar
(proposed by Tong) with a normal dipolar field ($\sim 10^{12}$ G) and
relatively strong multipole field. For the large luminosity variations of this
source, our scenario can also present a self-consistency interpretation.
