The invention relates to a composite base membrane for nano-filtration membrane separation and a preparation method of the composite base membrane, and belongs to the technical field of nano-filtration membranes. The base membrane comprises 15-20 parts of polyacrylonitrile, 40-50 parts of dimethylacetamide, and 40-50 parts of methyl ethyl ketone. The polyacrylonitrile base film is prepared from the polyacrylonitrile as the raw material of the base membrane as well as the dimethylacetamide and the methyl ethyl ketone as a mixed solvent, and the separating performance of the nano-filtration membrane prepared from the base membrane is excellent.