  NGC 300 ULX1 is a pulsating ultraluminous X-ray source (PULX) with the
longest spin period of $P\simeq31.6\,\rm s$ and a high spin-up rate of $\dot
P\simeq5.56\times10^{-7}\,\rm s\,s^{-1}$ that is ever seen in the confirmed
PULXs. In this paper, the inferred magnetic field of NGC 300 ULX1 is
$\sim3.0\times10^{14}\,\rm G$ using the recent observed parameters after its
first detection of pulsations. According to the evolved simulation of the
magnetic field and the spin period, it will become a recycled pulsar or a
millisecond pulsar under the conditions of the companion mass and the accretion
rate limitation. We suggest that NGC 300 ULX1 is an accreting magnetar
accounting for its super Eddington luminosity. We also propose that there might
be other accreting magnetars in the confirmed PULXs. Such PULXs will be helpful
for understanding the magnetar evolution and the millisecond pulsar formation
whose magnetic field is stronger than $\sim10^{9}\,\rm G$.
