  MAXI J1820+070 is a newly-discovered black hole X-ray binary, whose dynamical
parameters, namely the black hole mass, the inclination angle and the source
distance, have been estimated recently. \emph{Insight}-HXMT have observed its
entire outburst from 2018 March 14. In this manuscript, we fitted the spectra
obtained by \emph{Insight}-HXMT in the soft state to a fully-relativistic thin
disk model {\sc kerrbb2}, and found a slowly-spin black hole of
$a_*=0.13^{+0.07}_{-0.10}$ ($1\sigma$) with the continuum-fitting method.
