  We present the astrometric calibration of the Beijing-Arizona Sky Survey
(BASS). The BASS astrometry was tied to the International Celestial Reference
Frame via the \emph{Gaia} Data Release 2 reference catalog. For effects that
were stable throughout the BASS observations, including differential chromatic
refraction and the low charge transfer efficiency of the CCD, we corrected for
these effects at the raw image coordinates. Fourth-order polynomial
intermediate longitudinal and latitudinal corrections were used to remove
optical distortions. The comparison with the \emph{Gaia} catalog shows that the
systematic errors, depending on color or magnitude, are less than 2
milliarcseconds (mas). The position systematic error is estimated to be about
$-0.01\pm0.7$ mas in the region between 30 and 60 degrees of declination and up
to $-0.07 \pm 0.9$ mas in the region north of declination 60 degrees.
