According to single-photon quantum efficiency (SPQE) of single-photon avalanche photodiodes (SPADs), a strict mathematical model was proposed. The model is suitable for SPADs with In0.52 Al0.48 As or InP multiplication layer as well as In0.52 Al0.48 As-InP heterojunction multiplication layer for operating wavelengths of 1.3 μm and 1.5 μm. As the function of the device structure, operating voltage, and multiplication layers materials, the model can be used to optimize SPQE, furthermore evaluate and optimize the performance of APDs in Geiger mode.