A new extraction system, tetraoctylammonium bis(trifluoromethylsulfonyl)imide (N₈₈₈₈NTf₂)/tri-n-butyl phosphate (TBP)/sodium perchlorate (NaClO₄), is proposed for extracting lithium (Li) from magnesium (Mg)-rich solution. This system can increase the Li⁺ extraction efficiency compared with TBP/NaClO₄ system and decrease the stripping acidity compared with TBP/bis(trifluoromethylsulfonyl)imide lithium (LiNTf₂) system. Cation loss of ionic liquid can also be avoided in the extraction process. Under optimal conditions, the Li⁺ extraction efficiency reaches 82.3% and the Li⁺/Mg²⁺ separation factor reaches 254. The Li⁺ stripping efficiency reaches about 100% when 0.5 mol·L⁻¹ HCl is used as stripping agent. The extraction mechanism of N₈₈₈₈NTf₂/TBP/NaClO₄ system is proposed based on experimental results and verified by Fourier transform infrared spectroscopy (FT-IR), slope analysis, and simulation calculation using Gaussian software. N₈₈₈₈NTf₂/TBP/NaClO₄ system is highly stable over seven extraction cycles. This new extraction system has a great potential for selective extracting Li⁺ from Mg²⁺ rich brine.