  HD49798/RX J0648.0-4418 is a peculiar binary including a hot subdwarf of O
spectral type and a compact companion in a 1.55 day orbit. According to the
steady spin period derivative $\dot{P}=(-2.17\pm0.01)\times10^{-15} ~\rm
s\,s^{-1}$ , the compact object was thought to be a contracting young white
dwarf (WD). However, the X-ray luminosity producing by the wind accretion of
massive WD is one order of magnitude smaller than the observed value. In this
work, we propose an alternative model to account for the observed X-ray
luminosity. If the WD was surrounded by a debris disk, the accretion from the
debris disk can produce the observed X-ray luminosity and X-ray pulses. Based
on the time-varying accretion rate model, the current mass of the debris disk
is constrained to be $3.9\times10^{-6}~\rm M_{\odot}$. Comparing with the
contraction of the WD, the accretion torque exerting by such a debris disk can
only influence the spin evolution of the WD in the early stage. According to
the accretion theory, the magnetic field of the WD is constrained to be $\sim
(0.7-7)\times10^{4}$ G. The calculated conventional polar cap radius of the WD
is larger than the observed emitting-zone radius, which probably originate from
the existence of strong and small-scale local magnetic field in the polar cap
surface. We expect that further multiband observations on this source can help
us to confirm or rule out the existence of a debris disk.
