  GRO J1008-57, as a Be/X-ray transient pulsar, is considered to have the
highest magnetic field in known neutron star X-ray binary systems.
Observational data of the X-ray outbursts in GRO J1008-57 from 2017 to 2020
were collected by the Insight-HXMT satellite. In this work, the spin period of
the neutron star in GRO J1008-57 was determined to be about 93.28 seconds in
August 2017, 93.22 seconds in February 2018, 93.25 seconds in June 2019 and
93.14 seconds in June 2020. GRO J1008-57 evolved in the spin-up process with a
mean rate of $-(2.10\pm 0.05)\times$10$^{-4}$ s/d from 2009 -- 2018, and turned
into a spin down process with a rate of $(6.7\pm 0.6)\times$10$^{-5}$ s/d from
Feb 2018 to June 2019. During the type II outburst of 2020, GRO J1008-57 had
the spin-up torque again. During the torque reversals, the pulse profiles and
continuum X-ray spectra did not change significantly, and the cyclotron
resonant scattering feature around 80 keV was only detected during the
outbursts in 2017 and 2020. Based on the observed mean spin-up rate, we
estimated the inner accretion disk radius in GRO J1008-57 (about 1 - 2 times of
the Alfv\'{e}n radius) by comparing different accretion torque models of
magnetic neutron stars. During the spin-down process, the magnetic torque
should dominate over the matter accreting inflow torque, and we constrained the
surface dipole magnetic field $B\geq 6\times 10^{12}$ G for the neutron star in
GRO J1008-57, which is consistent with the magnetic field strength obtained by
cyclotron line centroid energy.
