We studied the relationship between the absorption layer length and the performance of Ge-on-Si microring resonator photodetectors. The principle of optimizing the absorption layer length based on the light field distribution was proposed. In the Ge-on-Si photodetectors, the transmission light field is alternately distributed among the germanium absorption layer and the silicon waveguide layer, and gradually absorbed by the germanium layer. For the Ge-on-Si microring resonator photodetectors, the length of the germanium absorption layer should be set to achieve the maximum light field distribution in the silicon layer at the end of the photodetector, then the remaining optical power can be coupled back to the silicon waveguide and transmit in the microring for absorption again. We demonstrated by simulation that, the device with optimized length of 11 mu m has larger bandwidth, smaller dark current, and higher responsivity than the device with 14 mu m absorption layer by simulation @1550nm.