  We present an analysis of the relativistic reflection spectra of GX 339-4
during the hard-to-soft transition of its 2021 outburst observed by
Insight-HXMT. The strong relativistic reflection signatures in the data suggest
a high black hole spin ($a_*>0.86$) and an intermediate disk inclination angle
(35-43 deg) of the system. The transition is accompanied by an increasing
temperature of the disk and a softening of the corona emission while the inner
disk radius remains stable. Assuming a lamppost geometry, the corona height is
also found to stay close to the black hole across the state transition. If we
include the Comptonization of the reflection spectrum, the scattering fraction
parameter is found to decrease during the state transition. We also perform an
analysis with a reflection model designed for hot accretion disks of stellar
mass black holes where the surface of the innermost accretion disk is
illuminated by emission from the corona and the thermal disk below. Our results
support the scenario in which the state transition is associated with
variations in the corona properties.
