  Based on HXMT observations of EXO 2030+375 during its 2021 giant outburst, we
report the analysis of pulse variations and the broadband X-ray spectrum, and
find the presence of a potential cyclotron resonant scattering feature with the
fundamental line at 47 keV from both average spectra and phase-resolved
spectroscopy. During the outburst, the source reached an X-ray luminosity of
$\sim 1\times 10^{38}$ erg /cm/s from 2-105 keV at a distance of 7.1 kpc. The
X-ray pulsar at the spin period of 41.27 seconds exhibits complex timing and
spectral variations with both energy and luminosity during the outburst. The
shapes of the pulses profiles show the single main peak above 20 keV, while
appear to exhibit multi-peak patterns in low energy bands, and the transition
of pulse profiles from multi-peak to single-peak is observed at $0.8\times
10^{38}$ erg /cm/s, which suggests the evolution from the subcritical
luminosity (pencil-beam dominated) to supercritical luminosity (fan-beam
dominated) regimes. A dip structure before the energy of the cyclotron resonant
scattering features is found in the pulse fraction-energy relation near the
peak luminosity. A detailed analysis of spectral parameters showed that the
power-law photon index exhibits three distinct trends as luminosity increases,
and these changes also signify a spectral transition from sub-critical to
super-critical regimes. The critical luminosity infers the magnetic field of
$(4.8-6.0)\times 10^{12}$ G, which supports the presence of the cyclotron line
at 47 keV. A Comptonization model applied for the broad X-ray spectra during
the outburst also suggests the surface magnetic field ranging from $(5-9)\times
10^{12}$ G.
