  The radioactive noble gas $^{222}$Rn, which can be dissolved in water, is an
important background source for JUNO. In this paper, based on the water system
of JUNO prototype, two kinds of high sensitivity radon detectors have been
proposed and developed. The sensitivity of Si-PIN Rn detector, which uses a
Si-PIN photodiode to detect the $\alpha$ from $^{214}$Po decay, is
$\sim$9.0~mBq/m$^3$. The sensitivity of LS Rn detector, which uses liquid
scintillator to detect the coincident signals of $\beta$ from $^{214}$Bi decay
and $\alpha$ from $^{214}$Po decay, is $\sim$64.0~mBq/m$^3$. Both of the two
kinds of Rn detector have the potential to be developed as an online Rn
concentration monitoring equipment for JUNO veto detector.
