  The Jiangmen Underground Neutrino Observatory (JUNO), a 20~kton multi-purpose
low background Liquid Scintillator (LS) detector, was proposed primarily to
determine the neutrino mass ordering. To suppress the radioactivity from the
surrounding rocks and tag cosmic muons, the JUNO central detector is submerged
in a Water Cherenkov Detector (WCD). In addition to being used in the WCD,
ultrapure water is used in LS filling, for which the $^{226}$Ra concentration
in water needs to be less than 50~$\mu$Bq/m$^3$. To precisely measure the
$^{226}$Ra concentration in water, a 6.0~$\mu$Bq/m$^3$ $^{226}$Ra concentration
in water measurement system has been developed. In this paper, the detail of
the measurement system as well as the $^{226}$Ra concentration measurement
result in regular EWII ultrapure water will be presented.
