  The magnetic field of ultraluminous X-ray (ULX) pulsars is the key parameter
to understand the nature and accretion physics. However, the typical magnetic
field values in these ULX pulsars are still under debate. We used six different
torque models to study the magnetic fields of ULX pulsars, to see how derived
magnetic fields change with different models, and to determine which models are
more suitable for ULX pulsars. We took the currently available period, period
derivative, and flux data of 7 confirmed ULX pulsars, M82 X-2, ULX NGC 7793
P13, ULX NGC 5907, NGC 300 ULX1, NGC 1313 X-2, M51 ULX-7, Swift J0243.6+6124,
plus one potential ULX pulsars, SMC X-3. The magnetic fields of these ULX
pulsars were constrained from two physical conditions: the spin-up process and
near equilibrium. We checked possible dependence of the magnetic field
estimations on the different torque models. The calculations suggest the
accretion torque models by Ghosh & Lamb [1], Wang [2], Kluzniak & Rappaport [3]
and Campbell [4] are more likely to support magnetar models for ULX pulsars,
while Lovelace, Romanova & Bisnovatyi-Kogan [5]'s model generally predicts the
magnetic field in normal neutron stars. Implications of our results combined
with other independent methods are also discussed, which will help us to
understand the nature and rotational behavior of these ULX pulsars.
