In order to depict the complex structure of the natural formations,the indicator function model was used and expressions for the mean, variance and covariance function of the medium parameters of the bimodal formations were derived. A quasi unimodal porous medium was generated. The spectral method based on Kahunen-Loeve expansion was applied in the stochastic analysis of the bimodal media. The water flow in bimodal formations and qausi unimdoal formation were simulated respectively, and the impact of the hydraulic conductivity and parameter α to equivalence between the bimodal and qausi unimdoal formation was analyzed. It was shown that the increase of the total variance of the variable parameters doesn’t lead to a bigger truncation order. If considering the spatial variability of the parameter Ks or α only, the accuracy of the hydraulic head from quasi unimodal formation becomes worse with the increase of the total variance of Ks or α; When Ks or α of the two mediums distinguishes obviously and has small variance, the quasi unimodal formation can substitute bimodal formations; The equivalence is affected more obviously by parameter α than Ks. If considering the spatial variability of the parameter Ks and α at the same time, the larger gap of Ks or α leads to a bad equivalence . The relation between the total variance of Ks or α and equivalence is obscure.
