The invention provides a synthesis method and a purpose of a lithium ion blotting membrane. The synthesis method comprises the following steps of adding polyvinylidene fluoride powder and polyvinylpyrrolidone into N,N'-dimethyl acetamide; performing mechanical stirring at the constant temperature; keeping the temperature unchanged; performing still standing; forming a PVDF base film through a phase conversion technology; putting the PVDF base film into de-ionized water to be stored; soaking the PVDF base film into a Tris-HCl solution; then, adding dopamine; performing mechanical oscillation at the room temperature; obtaining the PVDF base film with a polydopamine layer; adding 12-crown ether-4 and lithium chloride into methyl alcohol; soaking the PVDF base film with the polydopamine layer on the surface into the mixed solution; then, adding ethylene glycol dimethacrylate and azodiisobutyronitrile; forming a mixed solution; introducing insert gas; then, performing sealing; putting the materials into a constant-temperature water bath oscillator for reaction; eluting the membrane obtained after the reaction; obtaining the lithium ion blotting membrane. The material prepared by the method has the advantages of high selectivity on Li<+> and obvious separation effect.