Based on the principle of thin film optics, the theory of electric dipole radiation, and the principle of Fabry-Prot microcavity, an optical model is built for the tandem organic light emitting diode (OLED) with the structure of Glass/ITO/N, N0-bis(naphthalen-1-yl)-N, N0-bis (phenyl)-benzidine(NPB)/tris(8-hydroxyquinoli-ne) aluminum (Alq3)/molybdenum trioxide (MoO3)/NPB/Alq3/LiF/Al. The influence of the functional layer thickness on the light output intensity of device is systematically studied, the laws of light output intensity for tandem OLEDs under different thickness values of functional layer are obtained. This model and the results obtained in this paper can present an in-depth understanding of the working mechanism of tandem OLED and help ones fabricate high efficiency OLED.