The invention discloses a bacterial cellulose diaphragm and a preparation method thereof. The surfaces of nanofibers of the bacterial cellulose diaphragm are coated with a layer of polyionic liquid, and the polyionic liquid is one or the combination of more of imidazolium salt polyionic liquid, pyrrolidine salt polyionic liquid, piperidine salt polyionic liquid and quaternary ammonium salt polyionic liquid. The preparation method comprises the following steps: the bacterial cellulose diaphragm is sequentially immersed in aqueous polyionic liquid solution for 2 to 48 hours, immersed in precipitator for 30 minutes to 24 hours and immersed in lithium salt solution for 2 to 48 hours, finally, a hot press is utilized to press the bacterial cellulose diaphragm to be thin, and after drying, the polyionic liquid-coated bacterial cellulose diaphragm is obtained. The advantages of the invention are as follows: (1) the polyionic liquid-coated bacterial cellulose diaphragm has the advantages of high mechanical strength, good thermal stability, good lyophilic property, high porosity and high ionic conductivity, and can increase the cyclic stability and safety of lithium ion batteries; (2) the preparation method is simple, has low requirement on equipment, and is suitable for mass production.