The invention belongs to the field of a lithium ion battery, and particularly relates to a polymer diaphragm material for a lithium ion battery and a preparation method of the polymer diaphragm material. The preparation method comprises the following steps of enabling a carbon nanotube with carboxyl on the surface to have radiation-induced graft copolymerization with bacterial cellulose solution, then directly adding the polymer, dissolving and homogenizing the polymer to obtain a membrane polymer solution, and finally preparing the polymer lithium ion battery diaphragm compositely modified by bacterial cellulose which is grafted with the carbon nanotube through a solution membrane casting method. The invention provides the polymer lithium ion battery diaphragm compositely modified by utilizing bacterial cellulose to be grafted with the carbon nanotube and a preparation method of the polymer diaphragm material, and the obtained polymer diaphragm has the advantages of good ion conductivity, high liquid absorption rate and large mechanical strength.