The invention provides a preparation method of a battery diaphragm coating material. The preparation method comprises the following steps that: PVDF and a covalent organic framework material are dissolved in an organic solvent, and are uniformly stirred; the ratio of the PVDF to the covalent organic framework material to N-methyl pyrrolidone is 20:(70 to 85):(5 to 10). When the material is appliedto the preparation of a lithium ion battery diaphragm, the problem of metal dissolution which appears when a transition metal oxide is used as a positive electrode material can be effectively solved;and the prepared diaphragm can effectively adsorb metal ions and prevent the metal ions from being deposited to a negative electrode in a charging and discharging process, so that the cycle performance of the transition metal oxide positive electrode material is improved.