A revised model based on Aranovich-Donohue Theory was built in this paper and was used to indicate the adsorbing behaviors of monomers in nano slits. This work tried to establish a theoretical model and get the fundamental mechanisms of the in situ-monomer-intercalation process which is the most common production method of polymer-layer silicate intercalated nanocomposite. According to the results, the multilevel adsorption and 3D phase transition phenomenon in nano slits predicted by Aranovich-Donohue Theory do not exit in reality. They are probably aroused by system errors caused by simple mean field approximation treatments. The nano slits have obvious boundary effects on density profile distribution of monomers. When the pore size increase, the density differences will be diminished. The adsorbing amounts will go up sharply in a narrow low-bulk-concentration region and will decrease rapidly at most bulk-concentration-regions accompanied by the increasing of interaction energies of adsorbates. This should attribute to the capillary condensation phenomenon. The newly established model will be definitely helpful to reveal the fundamental mechanisms of in situ-monomer-intercalation process for polymer-layer silicate intercalated nanocomposite.