  Based on Insight-HXMT data, we report on the pulse fraction evolution during
the 2017-2018 outburst of the newly discovered first Galactic ultraluminous
X-ray source (ULX) Swift J0243.6+6124. The pulse fractions of 19 observation
pairs selected in the rising and fading phases with similar luminosity are
investigated. The results show a general trend of the pulse fraction increasing
with luminosity and energy at super-critical luminosity. However, the relative
strength of the pulsation between each pair evolves strongly with luminosity.
The pulse fraction in the rising phase is larger at luminosity below
$7.71\times10^{38}$~erg~s$^{-1}$, but smaller at above. A transition luminosity
is found to be energy independent. Such a phenomena is firstly confirmed by
Insight-HXMT observations and we speculate it may have relation with the
radiation pressure dominated accretion disk.
