Photomultiplier tubes (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 17,612 20-inch PMTs in its central detector to achieve an unprecedented energy resolution of 3%/sqrt(E). It’s critical to construct a reliable and accurate optical model to describe the angular and spectral responses of PMTs in their working media. Based on the optical theory of multilayer thin films and a GEANT4-based simulation toolkit, we establish a new optical model for the 20-inch PMTs used in JUNO, including the MCP-PMT manufactured by NNVT company and the dynode PMT manufactured by Hamamatsu company. In this model, both light interactions with the PMT window and optical processes inside PMTs are taken into consideration. This poster introduces the new PMT optical model and its related experimental tests.