  Protoclusters, the progenitors of galaxy clusters, trace large scale
structures in the early Universe and are important to our understanding of
structure formation and galaxy evolution. To date, only a handful of
protoclusters have been identified in the Epoch of Reionization (EoR). As one
of the rarest populations in the early Universe, distant quasars that host
active supermassive black holes are thought to reside in the most massive dark
matter halos at that cosmic epoch, and could thus potentially pinpoint some of
the earliest protoclusters. In this letter, we report the discovery of a
massive protocluster around a luminous quasar at $z=6.63$. This protocluster is
anchored by the quasar, and includes three [CII] emitters at $z\sim6.63$, 12
spectroscopically confirmed Ly$\alpha$ emitters (LAEs) at $6.54<z\le6.64$, and
a large number of narrow-band imaging selected LAE candidates at the same
redshift. This structure has an overall overdensity of
$\delta=3.3^{+1.1}_{-0.9}$ within $\sim35\times74$ cMpc$^2$ on the sky and an
extreme overdensity of $\delta>30$ in its central region (i.e., $R\lesssim2$
cMpc). We estimate that this protocluster will collapse into a galaxy cluster
with a mass of $6.9^{+1.2}_{-1.4}\times10^{15}~M_\odot$ at the current epoch,
more massive than the most massive clusters known in the local Universe such as
Coma. In the quasar vicinity, we discover a double-peaked LAE which implies
that the quasar has a UV lifetime greater than 0.8 Myrs and has already ionized
its surrounding intergalactic medium.
