  We report the phase-resolved spectral results of the first Galactic Pulsating
Ultra-Luminous X-ray source (PULX) Swift J0243.6+6124, modeling at its
2017-2018 outburst peak using data collected by the Hard X-ray Modulation
Telescope (Insight-HXMT). The broad energy coverage of Insight-HXMT allows us
to obtain more accurate spectral continuum to reduce the coupling of broad iron
line profiles with other components. We use three different continuum spectrum
models but obtain similar iron line results. For the first time, we detected
the pulse characteristics of the broad iron line in a PULX. The variation in
width and intensity of this iron line with $\sigma \sim 1.2-1.5$\,keV has a
phase offset of about 0.25 from the pulse phase. We suggest that the uneven
irradiation of the thick inner disk by the accretion column produces the
modulated variation of the broad iron line. In addition, the non-pulsed narrow
line is suggested to come from the outer disk region.
