Abstract: The structure of pillared montmorillonite is partially disordered layer lattice. Its basal spacings can be expanded to many possible values with different probabili-ties depending on the type and structure of the pillars or the preparation conditions. If a formula describing the X-ray diffraction intensity of partially disordered layer lattice as a function of basal spacings and corresponding probabilities is computa-tionally simulated with the experimental XRD pattern, the basal spacings and prob-abilities can thus be obtained. In this work, two of the polyoxyaluminum pillared montmorillonite samples with Al_(13) as main pillars were prepared. The simulated results show that most of the basal spacings are expanded to 1.97 nm with a probability of 0.65 corresponding to Al_(13) as pillars; some of the basal spacings are 1.5 nm with a probability of 0.25 corresponding to Al_6 as the most possible pillaring species; the rest ones are 1.1 nm containing water molecules only in the interlamellar region. The experimental results obtained from CEC, ICP etc. are used to calculate the distribution of the pillars in the interlayer zone. Subsequently, a three-dimensional structural model is built up. The agreement of the pore-volume between theoretical calculation and experimental result verifies the rationality of the structural model. Therefore, the basal spacing usually characterized by d_(001) which is calculated from Bragg equation actually is not exact.