Single photon detection technology is a very sensitive photoelectric detection technology that can detect the energy of a single photon. It is a weak signal detection technology widely used in laser ranging, quantum communication, laser radar 3D imaging, and other fields. Single photon avalanche diodes generally operate in Geiger mode, that is, the operating voltage is greater than the breakdown voltage. Photon detection efficiency refers to the probability that a photon incident on a device is converted into a macroscopic signal that can be detected. It is an important parameter to measure the detection ability of single photon avalanche diodes. InGaAs/InP single photon avalanche diodes can work in near infrared band, has the characteristics of high photon detection efficiency and good time jitter, in addition to small size, high stability, excellent anti-radiation performance, can realize large array imaging and other advantages, become one of the most promising near infrared single photon detectors. The planar back-illuminated InGaAs/InP single photon avalanche diodes designed in this paper adopt the absorption grading charge multiplication separation structure, in which the charge layer can control the electric field intensity of the multiplier layer to be high enough to produce avalanche breakdown, while the absorption layer electric field intensity is limited to a certain range to reduce the tunneling effect. The grading layer can reduce the accumulation of carriers at the heterogeneous interface. The P+ active region is formed by two Zn diffusion structures, the multiplication region is limited to the area below the deep diffusion. The shallow diffusion can effectively limit the edge electric field of the multiplication region without using the guard ring structure to avoid premature edge breakdown. The structure design and numerical simulation of InGaAs/InP single photon avalanche diodes are carried out by using TCAD software and selecting the appropriate physical model and parameters, and the corresponding electrical and optical parameters are obtained. Then aiming at the effect of avalanche breakdown probability on device photon detection efficiency, the relationship between the avalanche breakdown probability of the device and the difference between the two Zn diffusion depths, the lateral diffusion factor of Zn diffusion, the doping concentration of Zn, and the temperature parameters is emphatically studied. It is found that when the depth of deep diffusion is 2.3 μm, there is an optimal target value corresponding to the shallow diffusion depth. The deeper the shallow diffusion depth, the higher the breakdown probability of avalanche in the center of the multiplication region under the same overbias, and the higher the electric field intensity will also increase. However, when the difference between the two Zn diffusion depths is less than 0.6 μm, non-ideal breakdown will occur outside the multiplication region, resulting in an increase in the dark count of the device. The larger the lateral diffusion factor of Zn diffusion, the higher the breakdown probability of the avalanche at the center of the multiplication region, and the lower the breakdown probability of the avalanche at the edge of the multiplication region. With the same diffusion depth, the shallow diffusion Zn doping concentration has no significant effect on avalanche breakdown probability, but the higher the deep diffusion Zn doping concentration, the lower the avalanche breakdown probability under the same overbias. The study of the influence of temperature on avalanche breakdown probability shows that the device can obtain better performance at low temperature. The research work in this paper can guide the design of InGaAs/InP single photon avalanche diodes with higher photon detection efficiency and lower dark count. Moreover, relevant research can also provide a reference for selecting the optimal working point of the device to ensure the device works in the best state and realize the optimal application of the device.