The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose underground experiment and the largest liquid scintillator (LS) detector going for neutrino mass hierarchy, precise neutrino oscillation parameter measurement and studies of other rare processes which include but not limited to solar neutrino, geo-neutrino, supernova neutrinos and the diffuse supernova neutrinos background. The 20kt LS central detector (CD) of JUNO and 20" PMT are the keys of the whole facility. Parallel efforts and R&D activities are addressing different aspects. A prototype detector with similar design as future JUNO CD with different PMTs are settle down to study the future detector performance and develop possible analysis algorithms. We present here some R&D progress and pre-results about the prototype.