  The Jiangmen Underground Neutrino Observatory is building the world's largest
liquid scintillator detector with a 20 kt target mass and about 700 m
overburden. The total underground space of civil construction is about 300,000
m$^3$ with the main hall volume of about 120,000 m$^3$, which is the biggest
laboratory in the world. Radon concentration in the underground air is quite
important for not only human beings' health but also the background of
experiments with rare decay detection, such as neutrino and dark matter
experiments. The radon concentration is the main hall is required to be around
100 Bq/m$^3$. Optimization of the ventilation with fresh air is effective to
control the radon underground. To find the radon sources in the underground
laboratory, we made a benchmark experiment in the refuge room near the main
hall. The result shows that the radon emanating from underground water is one
of the main radon sources in the underground air. The total underground
ventilation rate is about 160,000 m$^3$/h fresh air with about 30 Bq/m$^3$
$^{222}$Rn from the bottom of the vertical tunnel after optimization, and
55,000 m$^3$/h is used for the ventilation in the main hall. Finally, the radon
concentration inside the main hall decreased from 1600 Bq/m$^3$ to around 100
Bq/m$^3$. The suggested strategies for controlling radon concentration in the
underground air are described in this paper.
