  We carried out deep mapping observations of the atomic hydrogen (HI) 21 cm
line emission in a field centered on the famous galaxy group Stephan's Quintet
(SQ), using the Five-hundred-meter Aperture Spherical Telescope (FAST) equipped
with the 19-Beam Receiver. The final data cube reaches an HI column density
sensitivity of $5 \sigma = 2.1\times 10^{17}$ cm$^{-2}$ per 20 km s$^{-1}$
channel with an angular resolution of $4'.0$. The discovery of a large diffuse
feature of the HI emission in the outskirt of the intragroup medium of SQ was
reported in a previous paper (Xu et al. 2022). Here we present a new study of
the total HI emission of SQ and the detection of several neighboring galaxies,
exploiting the high sensitivity and the large sky coverage of the FAST
observations. A total HI mass of $M_{\rm HI} = 3.48 \pm 0.35 \times 10^{10}\;
M_\odot$ is found for SQ, which is significantly higher than previous
measurements in the literature. This indicates that, contrary to earlier
claims, SQ is not HI deficient. The excessive HI gas is mainly found in the
velocity ranges of 6200 - 6400 km s$^{-1}$ and 6800 - 7000 km s$^{-1}$, which
was undetected in previous observations that are less sensitive than ours. Our
results suggest that the ``missing HI" in compact groups may be hidden in the
low-density diffuse neutral gas instead of in the ionized gas.
