Theoretical and experimental investigations on the dual-diffusion behaviors of zinc atoms in InGaAs/InP single photon avalanche photodiode (SPAD) are performed. A formula of Xj=k(t-t0)^0.5+c to quantitatively predict the diffusion depth is obtained by fitting the simulated twice diffusion depths based on a two-dimensional (2D) model. The 2D impurity morphologies and the one dimensional impurity profiles for the dual-diffused region are characterized by using the scanning electron microscopy and the secondary ion mass spectrometry as a function of the diffusion depth respectively. InGaAs/InP SPAD devices with different dual-diffusion conditions are also fabricated which show breakdown behaviors well consistent with the simulated results under the same junction geometries. Dark count rate measurements are carried out as well. These results demonstrate an effective prediction route for accurately control of the dual-diffused zinc junction geometry in InP based planar device processing.