Abstract:Neutrino physics is one of the most important topics of particle physics, with many mysteries. The breach of the Neutrino physics is beyond the standard model, and it's also a leading edge crossing with the particle physics, astrophysics and cosmology. In 2012, Dayabay neutrino experiment found a surprisingly large new neutrino oscillation mode, which makes the measurement of neutrino mass hierarchy and CP phase possible. Jiangmen Underground Neutrino Observatory(JUNO, originally called Daya Bay II)was approved by the Chinese Academy of Sciences and supported through the Strategic Priority Research Programme in 2013. The construction started in 2015, and the operation is expected to start in 2020. The main scientific goal of JUNO is to determine the neutrino mass hierarchy. It can also measure 3 out of 6 neutrino mixing parameters to a precision better than 1%, enabling the unitarity test of the neutrino mixing matrix in order to search for new physics. JUNO's science endeavor will extend beyond particle physics, covering astrophysics, earth science and cosmology by studying supernova neutrinos, geo-neutrinos, solar neutrinos, atmospheric neutrinos, proton decays and dark matter searches, reaching advanced level internationally in many fields.The JUNO detector is located in an underground laboratory with 700m overburden, 53 km from nearby reactor power plants. Its central detector is filled with 20 kton LAB based liquid scintillator. When neutrinos go through the detector, a very small part of them will interact with the liquid, producing scintillation light seen by 15000 surrounding 20" photomultiplier tubes(PMTs). The energy of incident neutrinos and the interaction vertex can be reconstructed based on the charge and timing information from PMTs. The energy resolution is roughly inverse proportional to the square root of detected number of photon electrons. To reach the expected sensitivity of mass hierarchy, the energy resolution has to be better than 3% at 1 MeV, corresponding to 1200 photon electrons per MeV, which is a much better performance than the state of the art detector such as BOREXINO and KamLAND. The technological challenges are new type of PMTs with high efficiency and highly transparent liquid scintillator. The water pool will shield the central detector from natural radioactivity in surrounding rocks. It also serves as a water Cherenkov detector after equipped with PMTs, to tag cosmic muons. There is another muon tracking detector on top of the water pool, used to improve muon detection efficiency and to get better muon tracking.