The consumption of lithium resources in China accounts for 40% of the global consumption, but its production capacity can only meet 20% of the industrial demand, and 80% of lithium needs to be imported. Therefore, it is urgent to study a new generation of lithium extraction technology. Nanofiltration (NF) membrane has the ability of selective ion separation, showing outstanding potential in the selective separation of Mg2+ and Li+. Commercial NF membrane is negatively charged, which is influenced by Donnan effect and electrostatic attraction, and its separation ability for positively charged ions is limited. In recent years, it has been found that positively charged NF membrane can effectively separate Mg2+ and Li+ with similar hydrate ion radii. Firstly, the ion transport theory and selective screening mechanism in NF membrane are introduced. Secondly, the research status and shortcomings of commercial NF membrane in the field of lithium extraction from salt lakes are summarized. Then, the influence of preparation methods of positively charged NF membrane (including the selection of amine monomer, doping of nano-materials, introduction of interlayer and surface grafting, etc.) on improving the selective separation performance of Mg2+ and Li+ was analyzed. Finally, the challenges and prospects of positively charged NF membrane in practical engineering applications are summarized.