The invention provides a preparation method of an ethylene propylene rubber/montmorillonite nano composite material. The material is prepared by mixing ethylene propylene rubber, nano montmorillonite and dicumyl peroxide in a weight ratio of 100: (1-10): (2-5). The method comprises the following steps: by adopting a chitosan strip-modified Na-montmorillonite to obtain good modified nano montmorillonite; then, uniformly mixing the raw materials in a certain weight ratio; adding into an open mill or a mixer to mix; then, putting on a press vulcanizer to tablet and mould; and finally, preparing the EPR/MMT nano composite material which has a relatively good reinforcing effect and is uniformly dispersed. The patent provides the preparation method of the uniformly dispersed nano material and the preparation method is suitable for the preparation field of various nano functional composite materials.