  The second brightest gamma-ray burst, GRB 230307A (with a duration $T_{90}$ ~
40 s), exhibited characteristics indicative of a magnetar engine during the
prompt emission phase. Notably, a suspected kilonova was identified in its
follow-up optical and infrared observations. Here we propose that the origin of
GRB 230307A is a neutron star-white dwarf (NS-WD) merger, as this could
naturally interpret the long duration and the large physical offset from the
center of its host galaxy. In the framework of such a NS-WD merger event, the
late-time kilonova-like emission is very likely to be powered by the spin-down
of the magnetar and the radioactive decay of $^{56}$Ni, rather than by the
decay of r-process elements as these heavy elements may not be easy to be
synthesized in a NS-WD merger. It is demonstrated that the above scenario can
be supported by our fit to the late-time observational data, where a mass of ~
$10^{-3} \ \rm M_{\odot}$ $^{56}$Ni is involved in the ejecta of a mass of ~
$0.1 \ \rm M_{\odot}$. Particularly, the magnetar parameters required by the
fit are consistent with those derived from the early X-ray observation.
