  GRB 160821B is a short gamma-ray burst (GRB) at redshift $z=0.16$, with a
duration less than 1 second and without detection of any "extended emission" up
to more than 100 seconds in both {\em Swift}/BAT and {\em Fermi}/GBM bands. An
X-ray plateau with a sharp drop 180 seconds after the BAT trigger was observed
with {\em Swift}/XRT. No supernova or kilo-nova signature was detected.
Assuming the central engine of this SGRB is a recently born supra-massive
magnetar, we can explain the SGRB as jet radiation and its X-ray plateau as the
internal energy dissipation of the pulsar wind as it spins down. We constrain
its surface magnetic field as $B_{\rm p}<3.12\times 10^{16}$ G and initial spin
period as $P_0< 8.5\times 10^{-3}$ seconds. Its equation of state is consistent
with the GM1 model with $M_{\rm TOV} \sim 2.37 M_\odot$ and ellipticity
$\epsilon<0.07$. Its gravitational wave (GW) radiation may be detectable with
the future Einstein Telescope, but is much weaker than the current
detectability limit of advanced-LIGO. The GW radiation of such an event would
be detectable by advanced-LIGO if it occurred at a distance of 100 Mpc
($z=0.023$).
