By measuring the distribution of impurity Zn in InP of planar-type InGaAs/InP Geiger mode Avalanche Photodiodes,the function of doping concentration with diffusion depth was fitted, and the optimum depth of etch well at different multiplication layer thickness as well as the best etching method were studied using ionization integral.The results show that the optimum etch well depth is varied with multiplication layer thickness when the thickness of top InP is constant.And when the multiplication layer thickness is about 1 μm, the well depth should be between 0.1 μm and 0.3 μm. Reactive ion etching can obtain a good etch well morphology, which is beneficial to the suppression of edge breakdown.