The invention relates to a porous dickite coated non-woven lithium ion battery diaphragm and a preparation method thereof, and belongs to the field of diaphragm materials. The method is as follows: firstly preparing porous dickite powder having through pores and a pore size in the range of 0.1 to 1 mu m, using the powder as a coating material to mixing with a binder, a modifier and water to prepare coating slurry, and coating the surface of a PET nonwoven fabric with the coating slurry by a dipping method to form a lithium ion battery composite diaphragm. The method utilizes the unique pore structure feature of porous dickite to construct a uniform macroporous structure on the surface of a PET non-woven fabric substrate, thereby improving the porosity of the diaphragm; by use of good affinity of the surface functional group of the dickite to an electrolyte, the electrolyte wettability of the composite membrane is improved, the internal resistance of a battery is reduced, and the chargeand discharge cycle stability of the lithium ion battery can be improved. The preparation method has simple preparation method, low equipment requirement, low raw material cost and strong market competitiveness.