The main goal of the JUNO experiment is to determine the neutrino mass ordering with a 20 kt liquid-scintillator detector. The 20-inch PMT is crucial as one of JUNO key instruments to realize an excellent energy resolution of at least 3% at 1 MeV. The knowledge on PMT's characterisation and feature is critical for detector performance understanding. Large pulses from PMT dark count such as from the flasher or others are one of the serious concerns for detector noise control. Focusing on the large pulses from 20-inch PMT dark count, this paper is trying to investigate the causes by measurements with a muon tagging system. It is found that the large pules of 20-inch PMT dark count is contributed mainly from muons hitting the PMT glass, and we have a preliminary understanding of the results with a simulation based on Geant4.