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Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST)19-beam tracking observational mode, high sensitivity and high-velocityresolution HI spectral lines have been observed toward the high-massstar-forming region G176.51+00.20. This is a pilot study of searching for HInarrow-line self-absorption (HINSA) toward high-mass star-forming regions wherebipolar molecular outflows have been detected. This work is confined to thecentral seven beams of FAST. Two HINSA components are detected in all sevenbeams, which correspond to a strong CO emission region (SCER; with a velocityof $\sim$ $-$18 km s$^{-1}$) and a weak CO emission region (WCER; with avelocity of $\sim$ $-$3 km s$^{-1}$). The SCER detected in Beam 3 is probablymore suitably classified as a WCER. In the SCER, the HINSA is probablyassociated with the molecular material traced by the CO. The fractionalabundance of HINSA ranges from $\sim 1.1 \times 10^{-3}$ to $\sim 2.6 \times10^{-2}$. Moreover, the abundance of HINSA in Beam 1 is lower than that in thesurrounding beams (i.e., Beams 2 and 4--7). This possible ring could be causedby ionization of HI or relatively rapid conversion from HI to H$_2$ in thehigher-density inner region. In the WCER (including Beam 3 in the SCER), theHINSA is probably not associated with CO clouds, but with CO-dark or CO-faintgas.

HI Narrow-Line Self-Absorptions Toward the High-Mass Star-Forming Region G176.51+00.20