Potential in electric double layer is a principal parameter of significant importance in researches on particle-particle interactions and availability of nutrient elements in soil. In this study, a new method was designed to determine surface potential and Stern potential. This method can used to describe Stern potential by taking into account polarization of the strong surface electric field and to obtain surface potential of soil particles by taking into account the effect of coupling of ion steric hindrance and polarization. Tests of the method reveal that only point charge H+ (no steric hindrance and polarization) can be used as signal transmission to characterize charged particle surface. The surface potential determined is 5 times as high as Stern potential. In the light of distribution of the potential, the potential declines linearly in the Stern layer, and follows the Boltzmann pattern in distribution in the diffuse layer, and the descent is much faster in the Stern layer than in the diffuse layer.