  Recently, FRB 190520B with the largest extragalactic dispersion measure (DM),
was discovered by the Five-hundred-meter Aperture Spherical radio Telescope
(FAST). The DM excess over the intergalactic medium and Galactic contributions
is estimated as $\sim 900$ pc cm$^{-3}$, which is nearly ten times higher than
other fast radio bursts (FRBs) host galaxies. The DM decreases with the rate
$\sim0.1$ pc cm$^{-3}$ per day. It is the second FRB associated with a compact
persistent radio source (PRS). The rotation measure (RM) is found to be larger
than $1.8 \times 10^{5} \mathrm{rad} ~\mathrm{m}^{-2}$. In this letter, we
argue that FRB 190520B is powered by a young magentar formed by core-collapse
of massive stars, embedded in a composite of magnetar wind nebula (MWN) and
supernova remnant (SNR). The energy injection of the magnetar drives the MWN
and SN ejecta to evolve together, and the PRS is generated by the synchrotron
radiation of the MWN. The magnetar has the interior magnetic field
$B_{\text{int}}\sim (2-4)\times 10^{16}$ G and the age $t_{\text{age}}\sim
14-22$ yr. The dense SN ejecta and the shocked shell contribute a large
fraction of the observed DM and RM. Our model can naturally explain the
luminous PRS, decreasing DM and extreme RM of FRB 190520B simultaneously.
