  We report our observations of very bright prompt optical and reverse shock
(RS) optical emission of GRB 140512A and analyze its multi-wavelength data
observed with the {\em Swift} and {\em Fermi} missions. It is found that the
joint optical-X-ray-gamma-ray spectrum with our first optical detection
(R=13.09 mag) at $T_0+136$ seconds during the second episode of the prompt
gamma-rays can be fit by a single power-law with index $-1.32\pm 0.01$. Our
empirical fit to the afterglow lightcurves indicates that the observed bright
optical afterglow with R=13.00 mag at the peak time is consistent with
predictions of the RS and forward shock (FS) emission of external shock models.
Joint optical-X-ray afterglow spectrum is well fit with an absorbed single
power-law, with an index evolving with time from $-1.86\pm 0.01$ at the peak
time to $-1.57\pm 0.01$ at late epoch, which could be due to the evolution of
the ratio of the RS to FS emission fluxes. We fit the lightcurves with standard
external models, and derive the physical properties of the outflow. It is found
that the ratio $R_B\equiv\epsilon_{\rm B, r}/\epsilon_{\rm B, f}$ is 8187,
indicating a high magnetization degree in the RS region. Measuring the relative
radiation efficiency with $R_e\equiv\epsilon_{\rm e, r}/\epsilon_{\rm e, f}$,
we have $R_e= 0.02$, implying the radiation efficiency of the RS is much lower
than that in FS. We also show that the $R_B$ of GRBs 990123, 090102, and
130427A are similar to that of GRB 140512A and their apparent difference may be
mainly attributed to the difference of the jet kinetic energy, initial Lorentz
factor, and medium density among them.
