The optical saturation characteristics in the germanium-on-silicon (Ge-on-Si) photodetector are studied for the first time, to the best of our knowledge. The relationship between the optical saturation characteristics and the optical field distribution in the Ge layer is illustrated by the simulation. This theory is verified by comparative experiments with single-injection and dual-injection structures. The dual-injection photodetector with a more balanced and uniform optical field distribution has a 13% higher responsivity at low optical power and 74.4% higher saturation current at 1550 nm. At higher optical power, the bandwidth of the dual-injection photodetector is five times larger than that of the single-injection photodetector.