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Ultrathin optical interference in a system composed of absorbing material and metal reflector has attracted extensive attention due to its potential application in realizing highly efficient optical absorption by using extremely thin semicon-ductor material. In this paper, we study the physics behind the high absorption of ultrathin film from the viewpoint of destructive interference and admittance matching, particularly addressing the phase evolution by light propagation and in-terface reflection. The physical manipulations of the ultrathin interference effect by controlling the substrate material and semiconductor material/thickness are examined. We introduce typical two-dimensional materials—i.e., MoS2 and WSe2—as the absorbing layer with thickness below 10 nm, which exhibits～90％absorption in a large range of incident angle (0°～70°). According to the ultrathin interference mechanism, we propose the ultrathin (<20 nm) MoS2/WSe2 hetero-junction for photovoltaic application and carefully examine the detailed optoelectronic responses by coupled multiphysics simulation. By comparing the same cells on SiO2 substrate, both the short-circuit current density (up to 20 mA/cm2) and the photoelectric conversion efficiency (up to 9.5％) are found to be increased by～200％.