The invention relates to a preparation method of a separator for a lithium sodium battery. The preparation method comprises the following steps of S1, dissolving 4,4-difluorobenzophenone and fluorene-9-bisphenol according to a mole ratio being (1:1)-(1:2) in a mixed solvent, and performing uniform stirring; S2, adding a catalyst under protection of inert gas, performing heating and heat preservation on a mixture to obtain a solid-state precipitant, and washing and drying the solid-state precipitant after being immersed with a solvent to prepare a separator material; and S3, dissolving the separator material in the solvent to form solutions with different concentrations, and preparing a polymer separator with a nanometer fiber mesh-shaped structure by an electrostatic spinning technology. The preparation method is easy to implement, the obtained lithium sodium ion battery polymer separator has excellent integral performance including high-temperature resistance, high porosity, good liquid absorption and maintenance performance, large ion conductivity and high mechanical strength, the separator has high application ceiling temperature, and the assembled button cell has favorable rateperformance.