  GRB 200219A is a short gamma-ray burst (GRB) with an extended emission (EE)
lasting $\sim 90$s. By analyzing data observed with the {\em Swift}/BAT and
{\em Fermi}/GBM, we find that a cutoff power-law model can adequately fit the
spectra of the initial short pulse with $\rm E_{p}=1387^{+232}_{-134}$ keV.
More interestingly, together with the EE component and early X-ray data, it
exhibits plateau emission smoothly connected with a $\sim t^{-1}$ segment and
followed by an extremely steep decay. The short GRB composed of those three
segments is unique in the {\em Swift} era and is very difficult to explain with
the standard internal/external shock model of a black hole central engine, but
could be consistent with the prediction of a magnetar central engine from the
merger of an NS binary. We suggest that the plateau emission followed by a
$\sim t^{-1}$ decay phase is powered by the spin-down of a millisecond
magnetar, which loses its rotation energy via GW quadrupole radiation. Then,
the abrupt drop decay is caused by the magnetar collapsing into a black hole
before switching to EM-dominated emission. This is the first short GRB for
which the X-ray emission has such an intriguing feature powered by a magnetar
via GW-dominated radiation. If this is the case, one can estimate the physical
parameters of a magnetar, the GW signal powered by a magnetar and the
merger-nova emission are also discussed.
