  We report on the Insight-HXMT observations of the new black hole X-ray binary
MAXI J1820+070 during its 2018 outburst. Detailed spectral analysis via the
continuum fitting method shows an evolution of the inferred spin during its
high soft sate. Moreover, the hardness ratio, the non-thermal luminosity and
the reflection fraction also undergo an evolution, exactly coincident to the
period when the inferred spin transition takes place. The unphysical evolution
of the spin is attributed to the evolution of the inner disc, which is caused
by the collapse of a hot corona due to condensation mechanism or may be related
to the deceleration of a jet-like corona. The studies of the inner disc radius
and the relation between the disc luminosity and the inner disc radius suggest
that, only at a particular epoch, did the inner edge of the disc reach the
innermost stable circular orbit and the spin measurement is reliable. We then
constrain the spin of MAXI J1820+070 to be a*=0.2^{+0.2}_{-0.3}. Such a slowly
spinning black hole possessing a strong jet suggests that its jet activity is
driven mainly by the accretion disc rather than by the black hole spin.
