  We present a detailed study of the high mass X-ray binary Vela X-1, using
observations performed by Insight-HXMT in 2019 and 2020, concentrating on
timing analysis and spectral studies including pulse phase-resolved
spectroscopy. The cyclotron line energy is found to be at ~21-27 keV and 43-50
keV for the fundamental and first harmonic, respectively. We present the
evolution of spectral parameters and find that two line centroid energy ratio
E2/E1 evolved from ~2 before MJD 58900 to ~1.7 after that. The harmonic
cyclotron line energy has no relation to the luminosity but the fundamental
line energy shows a positive correlation with X-ray luminosity, suggesting that
Vela X-1 is located in the sub-critical accreting regime. In addition, the
pulse phase-resolved spectroscopy in Vela X-1 is performed. Both the CRSF and
continuum parameters show strong variability over the pulse phase with the
ratio of two line energies about 2 near the peak phases, and down to ~1.6
around off-peak phases. Long-term significant variations of the absorption
column density and its evolution over the pulse phase may imply the existence
of the clumpy wind structure near the neutron star.
