The variation of the current characteristics of a Si-based Ge PIN infrared detector with the strain, thickness and doping concentration of a Ge absorption layer is numerically simulated on the basis of a drift-diffusion model and an optimized device design scheme is given. The research result shows that when the tensile strain of the Ge absorption layer is increased from 0 to 0.3%, the dark current of the device is increased by about 50%; when the thickness of the Ge absorption layer is increased from 1μm to 4μm, the dark current of the device is decreased by about 80% and its quantum efficency is nearly doubled; and when the doping concentration of the Ge absorption layer is increased from 1×1014cm-3 to 1×1016cm-3, the dark current of the device is decreased by about 60%. In view of the effect of absorption layer thickness on the quantum efficiency and dark current of the device and the effect of doping concentration of the absorption layer on photocurrent, the Ge absorption layer of the Si-based Ge PIN infrared detector is designed to have a thickness of 4μm and a doping concentration of 1×1014cm-3. It is expected that this design can provide a good basis for the improvement of device performance and the fabrication of practical devices.