To explore the feasibility of extracting lithium metal from brine sources, three salts, MgCl2, CaCl2, and NH4Cl, were selected as chloride sources, and the extraction equilibrium of lithium was studied with tributyl phosphate (TBP) in kerosene and FeCl3 as a coextracting agent at different values of Fe/Li. The extraction mechanism for lithium with TBP in kerosene and FeCl3 as a coextracting agent was investigated too. The results showed that the extraction of the lithium ion is a cation-exchange reaction, and the extraction of iron ion is the precondition of the extraction of lithium ion. All of the extractability of the iron ion increased with the chloride concentration with MgCl2, CaCl2, and NH4Cl as chloride sources, and the extraction capacity of lithium ion followed the sequence: MgCl2 > CaCl2 > NH4Cl, with recoveries from MgCl2 as chloride sources being much higher than that for CaCl2 and NH4Cl as chloride sources at all values of Fe/Li. There exists competitiveness between Li+ and NH4+, Ca2+, and Mg2+ when combined to TBP and FeCl3 and a salting-out effect of three salts. MgCl2 benefits from weaker competitiveness and a stronger salting-out effect than the other two. Choosing MgCl2 as chloride sources at Fe/Li = 1.9 obtains the highest partition coefficient with TBP/kerosene as an extractant and FeCl3 as a coextracting agent.