  Using the HI self-absorption data from the Five-hundred-meter Aperture
Spherical radio Telescope (FAST), we perform a study of the cold atomic gas in
the Cygnus-X North region. The most remarkable HI cloud is characterized by a
filamentary structure, associated in space and in velocity with the principle
molecular filament in the Cygnus-X North region. We investigate the transition
from the atomic filament to the molecular filament. We find that the HII
regions Cygnus OB2 and G081.920+00.138 play a critical role in compressing and
shaping the atomic Cygnus-X North filament, where the molecular filament
subsequently forms. The cold HI in the DR21 filament has a much larger column
density (N(HI) $\sim$ 1 $\times$ 10$^{20}$ cm$^{-2}$) than the theoretical
value of the residual atomic gas ($\sim$ 1 $\times$ 10$^{19}$ cm$^{-2}$),
suggesting that the HI-to-H$_2$ transition is still in progress. The timescale
of the HI-to-H$_2$ transition is estimated to be 3 $\times$ 10$^{5}$ yr, which
approximates the ages of massive protostars in the Cygnus-X North region. This
implies that the formation of molecular clouds and massive stars may occur
almost simultaneously in the DR21 filament, in accord with a picture of rapid
and dynamic cloud evolution.
