Recently, photoemission spectroscopy has been widely used for the study of quantum well states (QWS) in films, several models have been developed to explain the relationship of the discrete quantum well states and photoemission spectroscopy. Three theoretical models, nearly free electron model, phase accumulation model, and electron interferometer model, are introduced. Using these models, linewidth and energy position of peaks in photoemission spectroscopy can be explained. The linewidth is determined by the inverse lifetime г of quasiparticle and the reflectivities R at the surface and interface, while the peak position is determined by the wave vector k and total phase shift Ф due to reflection at the surface and interface.