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We present a spectral study of the ultraluminous Be/X-ray transient pulsarSwift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER)observations during the system's 2017--2018 giant outburst. The 1.2--10~keVenergy spectrum of the source can be approximated with an absorbed cut-offpower law model. We detect strong, luminosity-dependent emission lines in the6--7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddingtonregime, is seen to evolve into a broad Fe-line profile in the super-Eddingtonregime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex.An asymmetric broad line profile, peaking at 6.67 keV, is possibly due toDoppler effects and gravitational redshift. The 1.2--79 keV broadband spectrumfrom NuSTAR and NICER observations at the outburst peak is well described by anabsorbed cut-off power law plus multiple Gaussian lines and a blackbodycomponent. Physical reflection models are also tested to probe the broad ironline feature. Depending on the mass accretion rate, we found emission sitesthat are evolving from ~5000 km to a range closer to the surface of the neutronstar. Our findings are discussed in the framework of the accretion disk and itsimplication on the magnetic field, the presence of optically thick accretioncurtain in the magnetosphere, jet emission, and the massive, ultra-fast outflowexpected at super-Eddington accretion rates. We do not detect any signatures ofa cyclotron absorption line in the NICER or NuSTAR data.

An evolving broad iron line from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124