  We construct a disk-corona model which accounts for the optical-to-X-ray
spectral properties of Seyfert nuclei and QSOs. We study emission spectrum
emerging from a vertical disk-corona structure composed of two-temperature
plasma by solving hydrostatic equilibrium and radiative transfer
self-consistently. A fraction $f$ of viscous heating is assumed to be
dissipated in a corona with a Thomson optical depth of $\tau_c$, where
advective cooling is also included, and a remaining fraction, $1-f$, dissipates
within a main body of the disk. Our model can nicely reproduce the soft X-ray
excess with a power-law shape and the hard tail extending to $\sim$ 50 keV. The
different spectral slopes ($\alpha \sim$ 1.5 below 2keV and $\sim$ 0.5 above,
where $\Fnu \propto \nu^{- \alpha}$) are the results of different emission
mechanisms and different sites; the former slope is due to unsaturated
Comptonization from the innermost zone and the latter is due to a combination
of the Comptonization, bremsstrahlung and a reflection of the coronal radiation
at the disk-corona boundary from the inner to surrounding zone ($\leq$ 300
Schwarzschild radii). The emergent optical spectrum is redder ($\alpha \sim
0.3$) than that of the standard disk ($\alpha \sim -0.3$), being consistent
with observations, due to the different efficiencies of spectral distortion of
disk emission at different radii. Further, we find that the cut-off frequency
of the hard X-ray ($\sim$ coronal electron temperature) and broad-band spectral
shape are insensitive to the black-hole mass, while the peak frequency of the
big blue bump is sensitive to the mass as the peak frequency $\propto
\Mbh^{-1/4}$.
