  The Jiangmen Underground Neutrino Observatory (JUNO) is an experimental
project designed to determine the neutrino mass ordering and probe the
fundamental properties of the neutrino oscillations. The JUNO central detector
is a spherical liquid scintillator detector with a diameter of 35.4 m and
equipped with approximately 18,000 20-inch PMTs. A trigger threshold of 0.5 MeV
can be easily achieved by using a common multiplicity trigger and can meet the
requirements for measuring neutrino mass ordering. However, it is essential to
further reduce the trigger threshold for detecting solar neutrinos and
supernova neutrinos. A sophisticated trigger scheme is proposed to achieve a
low energy threshold by reducing the level of low energy radioactivity and dark
noise coincidence. With the new trigger scheme, the events rate of the central
detector from different types of sources have been carefully studied by using a
detailed detector simulation. It shows that the trigger threshold can be
reduced to 0.2 MeV, or even 0.1 MeV, if the concentration of $^{14}$C in liquid
scintillator can be well controlled.
