  We report the evolution of the X-ray pulsations of EXO 2030+375 during its
2021 outburst using the observations from \textit{Insight}-HXMT. Based on the
accretion torque model, we study the correlation between the spin frequency
derivatives and the luminosity. Pulsations can be detected in the energy band
of 1--160 keV. The pulse profile evolves significantly with luminosity during
the outburst, leading to that the whole outburst can be divided into several
parts with different characteristics. The evolution of the pulse profile
reveals the transition between the super-critical (fan-beam dominated) and the
sub-critical accretion (pencil-beam dominated) mode. From the accretion torque
model and the critical luminosity model, based on a distance of 7.1 kpc, the
inferred magnetic fields are $(0.41-0.74) \times 10^{12}$ G and $(3.48-3.96)
\times 10^{12}$ G, respectively, or based on a distance of 3.6 kpc, the
estimated magnetic fields are $(2.4-4.3) \times 10^{13}$ G and
$(0.98-1.11)\times 10^{12}$ G, respectively. Two different sets of magnetic
fields both support the presence of multipole magnetic fields of the NS.
