MAXI J1820+070 is a low-mass black hole X-ray binary system with high luminosity in both optical and X-ray bands during its outburst periods. We present extensive photometry in the X-ray, ultraviolet, and optical bands, as well as densely cadenced optical spectra, covering the phase from the beginning of the optical outburst to similar to 550 days. During the rebrightening process, the optical emission precedes the X-ray by 20.80 +/- 2.85 days. The spectra are characterized by blue continua and emission features of Balmer series, He I, and He II lines and a broad Bowen blend. The pseudo equivalent widths (pEWs) of emission lines are found to show anticorrelations with the X-ray flux measured at comparable phases, due to increased suppression by the optical continuum. At around the X-ray peak, the full widths at half-maximum (FWHMs) of H beta and He II lambda 4686 tend to stabilize at 19.4 and 21.8 angstrom, corresponding to the line-forming regions at radii of 1.7 and 1.3 R-circle dot within the disc. We further analysed 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, favouring the suggestion that the irradiation model is responsible for the optical emission. However, the fact that X-ray emission experiences a dramatic flux drop at t similar to 200 days after the outburst, while the optical flux only shows a little variation, suggests that additional energy such as viscous energy may contribute to the optical radiation, in addition to X-ray irradiation.