Resonant cavity enhanced Ge film photodiode on Si substrate is proposed, and the effect of the pair value of Si/SiO2 forming the top and bottom mirrors, thickness of the Ge absorption film, and mesa area of the active zone on the characteristics of the device such as external quantum efficiency and bandwidth is theoretically calculated. The optimized structure is as follows: the pair value of Si/SiO2 is 2 and 3 for top and bottom mirrors, respectively, the thickness of Ge film is 0.46 μm and the mesa area is less than 176 μm2 . Under this condition, an external quantum efficiency of 0.64 at a wavelength of 1.55 μm , which is 30 times larger than the conventional one, and a bandwidth of 40 GHz are achieved.