The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multi-purpose low background liquid scintillator detector, was proposed primarily to determine the neutrino mass ordering. For the sake of suppressing the radioactivity from the surrounding rocks and tagging the cosmic muons, the central detector is submerged in a water Cherenkov detector filled with 35 kton ultrapure water. Strict requirements are put forward for intrinsic radioactivity in water, i.e., the radon concentration should be less than 10 mBq/m3. In this poster, the ultrapure water system, the radon removal system and the online radon monitor system will be introduced.