The neutrino detector for the Jiangmen Underground Neutrino Observatory (JUNO) requires a large number of photomultiplier tubes (PMTs),including 15000 MCP PMTs and 5000 dynode PMTs.The TTS (transit time spread) of the PMTs is very important for vertex and track reconstruction of the neutrinos in the detector.In this paper,we study the TTS of a 20-inch dynode PMT (R12860) from Hamamatsu for different high voltage,light intensity,light spot size and different photocathode regions.The impact from Earth's magnetic field is also studied.The results achieved in this paper will be very useful for the JUNO experiment.