While massive quiescent galaxies are known to be poor in atomic hydrogen (HI), their true HI content remains poorly constrained due to the limited sensitivity and morphological biases of existing surveys. We present deep HI observations using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) for a representative sample of 78 low-redshift massive quiescent galaxies, selected by stellar mass ($M_\ast > 10^{10} M_\odot$) and color (NUV$-r > 5$). Our observations reach an exceptional detection limit of $\log(M_{HI}/M_\ast) = -3$. We find that one-third of the sample follows the predicted HI mass fraction distribution based on previous surveys. However, the majority ($\sim$2/3) exhibit significantly lower HI content, with a $3σ$ upper limit of $\log(M_{HI}/M_\ast) < -3.46$ derived from stacking the non-detections. As a consequence of this enormous dynamic range and the high fraction of non-detections, the HI mass fraction shows no clear correlation with parameters tracing star formation, structure, or morphology. Our FAST sample shows remarkable similarity to the $ATLAS^{3D}$ sample which only includes early-type galaxies, both in its high fraction of HI-poor galaxies and its high satellite fraction among HI-poor galaxies, as indicated by three different environmental characteristics: projected cross-correlation functions, background-subtracted neighbour counts, and central/satellite classification in the SDSS galaxy group catalog. These results suggest that while early-type morphology and environment play crucial roles in suppressing HI reservoirs, neither factor alone fully explains the observed gas depletion, indicating that additional physical mechanisms must be responsible for the extreme HI deficiency prevalent in massive quiescent galaxies.
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