A series of sodium bentonites and cetyl tri methyl ammonium bromide as raw materials were prepared to the modified organic composite montmorillonites. The performance characterization of the modified organic bentonite was observed by means of IR spectroscopy, X-ray diffraction, scanning electron microscopy. And the modified organic bentonite was applied into the pretreatment of landfill leachate, which was with the COD as high as 20000-37000 mg/L. The COD removal rate was invested as index. The optimum dosage and the best reaction conditions of bentonite have also been studied. The results indicated that the treatment effect of COD on the modified bentonite was better than the natural bentonite. Under the optimum conditions, the removal rate of COD reached 67.85% and the load of biological treatment was reduced on the next step.