  We present the analysis of a new near-infrared (NIR) spectrum of a recently
discovered $z=6.621$ quasar PSO J006+39 in an attempt to explore the early
growth of supermassive black holes (SMBHs). This NIR (rest-frame ultraviolet,
UV) spectrum shows blue continuum slope and rich metal emission lines in
addition to Ly$\alpha$ line. We utilize the MgII line width and the rest frame
luminosity $L_\text{3000\AA}$ to find the mass of SMBH ($M_\text{BH}$) to be
$\sim 10^8 M_\odot$, making this one of the lowest mass quasars at high
redshift. The power-law slope index ($\alpha_\lambda$) of the continuum
emission is $-2.94\pm0.03$, significantly bluer than the slope of
$\alpha_\lambda=-7/3$ predicted from standard thin disc models. We fit the
spectral energy distribution (SED) using a model which can fit local SMBHs,
which includes warm and hot Comptonisation powered by the accretion flow as
well as an outer standard disc. The result shows that the very blue slope is
probably produced by a small radial ($\sim230$ gravitational radius,
$R_\text{g}$) extent of the standard accretion disc. All plausible SED models
require that the source is super-Eddington ($L_\text{bol}/L_\text{Edd} \gtrsim
9$), so the apparently small disc may simply be the inner funnel of a puffed up
flow, and clearly the SMBH in this quasar is in a rapid growth phase. We also
utilize the rest-frame UV emission lines to probe the chemical abundance in the
broad line region (BLR) of this quasar. We find that this quasar has super
solar metallicity through photoionization model calculations.
