Abstract: When the n+-p-π-p⁺ reach-through silicon avalanche photodiodes (Si-RAPD) was fabricated by the ion implantation technique, it electric field can be obtained by means of the Gassian type impurity profile model. The relation between the multiplication factor M and the voltage V was found. The effects of the electric field strength on the breakdown voltage V_B, the depletion layer width W, and the effective ionization coneffioient K_eff are discussed in detail. The theoretically calculated results are in good agreement with experimentally measured values. By means of this model, it will be quite convenient to carried out the optimum design of the Si-RAPD.