In this paper, we theoretically study the performance of a separate absorption, grading, charge, and multiplication (SAGCM) InGaAs/InP single photon avalanche diode (SPAD) using a twodimensional drift-diffusion model based on our experimental results. The electric field, dark current and breakdown voltage are calculated for the SPAD for different thickness of the charge and the multiplication layers, to optimize the SPAD preformance. Moreover, we find that the number of InGaAsP grading layers could be decreased for the SPAD which is operated in Geiger mode, to keep a good dark current behavior in device.