A nano-class polyolefin composition is prepared through drying the montmorillonite, preparing montmorillonite carrier, adding it to solution of magnesium compound in alcohol, adding the mixed solution to the solution of Ti compound, adding acid anhdyride, reacting with Ti compound, filtering, hot washing with anhydrous non-polar solvent, drying to obtain catalyst, sequentially adding anhydrous hexane or pentane, the heptane solution of aluminium compound, organosilicon and said catalyst in reactor under atmosphere of ethylene or propylene monomer, polymerizing, and adding the acidified alcohol to stop reaction.