The GexSi1-x/Si strained-layer superlattices avalanche photodetector has been studied and optimum designed. Its optimum structure parameters are: the thickness of i-Si avalanehe region is 1. 8~2 μm; the carrier concentration of p-Si region is 10 18 cm-3 and thickness is 17 nm; the total thickness of superlattices is 340 nm. The detector can be operated in the range of 1.3~1.6 μm.