  MAXI J1820+070 is a low-mass black hole X-ray binary system with high
luminosity in both optical and X-ray bands during the outburst periods. We
present extensive photometry in X-ray, ultraviolet, and optical bands, as well
as densely-cadenced optical spectra, covering the phase from the beginning of
optical outburst to $\sim$550 days. During the rebrightening process, the
optical emission preceded the X-ray by 20.80 $\pm$ 2.85 days. The spectra are
characterized by blue continua and emission features of Balmer series, He I, He
II lines and broad Bowen blend. The pseudo equivalent width (pEW) of emission
lines are found to show anticorrelations with the X-ray flux measured at
comparable phases, which is due to the increased suppression by the optical
continuum. At around the X-ray peak, the full width at half maximums (FWHMs) of
H$_{\beta}$ and He II $\lambda$4686 tend to stabilize at 19.4 Angstrom and 21.8
Angstrom, which corresponds to the line forming region at a radius of 1.7 and
1.3 R_sun within the disk. We further analyzed the absolute fluxes of the lines
and found that the fluxes of H$_{\beta}$ and He II $\lambda$4686 show positive
correlations with the X-ray flux, favoring that the irradiation model is
responsible for the optical emission. However, the fact that X-ray emission
experiences a dramatic flux drop at t$\sim$200 days after the outburst, while
the optical flux only shows little variations suggests that additional energy
such as viscous energy may contribute to the optical radiation in addition to
the X-ray irradiation.
