Photomultipliers (PMTs) are widely used in scintillation and Cherenkov detectors for their great performances on photon detection. For example, the Jiangmen Underground Neutrino Observatory (JUNO) will use ~18,000 20-inch PMTs in its central detector to achieve an unprecedented energy resolution of 3%/√(E(MeV)). A key parameter of PMT is its detection efficiency (DE), which will be strongly influenced by the optical processes in PMT including: (1) Reflection between external medium and PMT bulb. (2) Reflection, absorption and transmission on its anti-reflective coating and photocathode, in which interference effect happens. (3) Multiple reflection inside PMT. Based on multilayer thin film theory, experimental tests and Geant4 simulations, a PMT optical model has been established for the 20-inch PMTs used in JUNO, including the NNVT MCP-PMT and the Hamamatsu dynode PMT. In this talk, the PMT optical model and its related experimental tests will be introduced.