The invention relates to the technical field of a battery, particularly a preparation method of a polymer electrolyte membrane. The preparation method comprises the following steps of (1) dissolving alow polyether grafted polyphosphazene polymer into a solvent to prepare into a substrate material solution with mass percentage concentration of 3-12%; (2) enabling a lithium salt and an inorganic filler to be dispersed into the substrate material solution to obtain a dispersed solution, wherein the mass percentage of the lithium salt in the dispersed solution is 10-15%; and (3) pouring the dispersed solution on a diaphragm to form a film in a curtain coating manner, and performing vacuum drying to obtain the low polyether grafted polyphosphazene-based polymer electrolyte membrane, wherein the vacuum drying temperature is 70-100 DEG C. The polymer electrolyte membrane prepared in the invention can be used as the polymer electrolyte membrane of a lithium ion battery, so that safety performance of the lithium ion battery is improved; and meanwhile, the obtained polymer electrolyte membrane has relatively high elasticity and processing performance, and mechanical performance.