  We present 13-band CCD intermediate-band spectrophotometry of a field
centered on the open cluster NGC 7789 from 400 to nearly 1000 nm, taken with
Beijing-Arizona-Taiwan-Connecticut (BATC) Multi-Color Survey photometric
system. By comparing observed spectral energy distributions of NGC 7789 stars
with theoretical ones, the fundamental parameters of this cluster are derived:
an age of $1.4\pm0.1$ Gyr, a distance modulus $(m-M)_{0}=11.27\pm0.04$, a
reddening $E(B-V)=0.28\pm0.02$, and a metallicity with the solar composition
$Z=0.019$. When the surface density profile for member stars with limiting
magnitudes of 19.0 in the BATC $e$ band ($\lambda_{\textrm{eff}}=4925$ \AA) is
fitted by King model, the core radius $R_{c}=7.52\arcmin$ and the tidal radius
$R_{t}=28.84\arcmin$ are derived for NGC 7789. The observed mass function (MF)
for main sequence stars of NGC 7789 with masses from 0.95 to 1.85 $M_{\odot}$
is fitted with a power-law function $\phi(m)\propto m^{\alpha}$ and the slope
$\alpha=-0.96$ is derived. Strong mass segregation in NGC 7789 is reflected in
the significant variation of the concentration parameters $C_{0}=\log
(R_{t}/R_{c})$ for member stars of NGC 7789 within different mass ranges:
$C_{0}=1.02$ for most of massive stars; $C_{0}=0.37$ for the lowest-mass MS
stars. Strong mass segregation in NGC 7789 is also indicated in the significant
variation of the slopes $\alpha$ in different spatial regions of the cluster:
the MF for stars within the core region has $\alpha=-0.71$, much flatter than
that for stars in external regions of the cluster ($\alpha=-1.20$).
