  We report on the spectral behavior of the first Galactic ultraluminous X-ray
pulsar Swift J0243.6+6124 with NuSTAR observations during its 2017-2018
outburst. At sub-Eddington levels, the source spectrum is characterized by
three emission components, respectively from the accretion column, the hot
spot, and a broad iron line emission region. When the source is above the
Eddington limit, the hot spot temperature increases and the spectrum features
two more blackbody components. One blackbody component has a radius of 10-20 km
and is likely originated from the top of the accretion column. The other one
saturates at a blackbody luminosity of (1 - 2)*10^38 erg/s, coincident with the
Eddington limit of a neutron star. This is well consistent with the scenario
that super-Eddington accretion onto compact objects will power optically-thick
outflows and indicates an accretion rate 60-80 times the critical value. This
suggests that super-Eddington accretion onto magnetized systems can also power
massive winds. At super-Eddington levels, the iron line becomes more
significant and blueshifted, and is argued to be associated with the ultrafast
wind in the central funnel or jets. This source, if located in external
galaxies, will appear like other ultraluminous pulsars.
