The invention relates to a material special for preparing a lithium ion battery diaphragm and a preparation method thereof. The material consists of the following components in part by weight: 62.5 to 86 parts of polyethylene, 0.5 to 1.5 parts of di-tert-butyl peroxide, 3 to 5 parts of vinyl triethoxy silane, 10 to 30 parts of titanium dioxide whiskers and 0.5 to 1 part of antioxidant. The preparation method comprises the following steps of: mixing the polyethylene, the di-tert-butyl peroxide, the antioxidant and the vinyl triethoxy silane, performing melt blending by using a double-screw extruder and performing an in-situ grafting reaction to obtain silane-grafted polyethylene; and stirring and blending the silane-grafted polyethylene and the titanium dioxide whiskers to obtain the material special for preparing the lithium ion battery diaphragm. The vinyl triethoxy silane is grafted on a polyethylene molecular chain and an ethyoxyl silane group can further react with the titanium dioxide whiskers so that a cross-linking reaction is realized in an entire membrane system and the intensity of a polyethylene membrane is enhanced.