In this work, we put forward the length design principle of the dual-injection integrated Ge-on-Si photodetectors based on the characteristics of the light field distribution. When the length of the absorption layer makes the maximum intensity of the incident light from both ends are alternately distributed, the light field distribution is more uniform. Based on this principle, the shorter photodetector could get better saturation performance. Two dual-injection photodetectors with the length of 6μm and 10μm have been investigated, the experimental results show that the saturation current of 6μm length device is 18.33% higher, and the bandwidth is 138.60% higher at 10 mW incident power comparing with the 10μm length device. The 6μm length photodetector realized a responsivity of 1.07 A/W, a bandwidth of 37.3 GHz at 50 μW luminous power @1550 nm. © 2020 Elsevier B.V.