  A fast radio burst (FRB) was recently detected to be associated with a hard
X-ray burst from the Galactic magnetar SGR 1935+2154. Scenarios involving
magnetars for FRBs are hence highly favored. In this work, we suggest that the
impact between an asteroid and a magnetar could explain such a detection.
According to our calculations, an asteroid of mass $10^{20}$ g will be
disrupted at a distance of $7 \times 10^9$ cm when approaching the magnetar.
The accreted material will flow along the magnetic field lines from the
Alfv\'en radius $\sim 10^7$ cm. After falling onto the magnetar's surface, an
instant accretion column will be formed, producing a Comptonized X-ray burst
and an FRB in the magnetosphere. We show that all the observational features of
FRB 200428 could be interpreted self-consistently in this scenario. We predict
quasi-periodic oscillations in this specific X-ray burst, which can serve as an
independent observational test.
