It is important to find an effective way to separate lithium from the mother liquor obtained by the reaction of lithium chloride/lithium sulfate and sodium carbonate. A synergistic solvent extraction system containing 1-phenyl-3-heptyl-1,3-propanedione (PHPD) and liquid mixture of trialkyl-phosphine oxides (Cyanex923) to separate lithium from the system of Li++Na++Cl-+CO32-+H2O was developed. The pH of the aqueous solution showed an important effect on the lithium extraction. At pH 13.06, PHPD alone showed a high extractability of lithium, but the addition of Cyanex923 led to a high separation ability of lithium over sodium due to the synergistic extraction effect. For a single extraction, the extraction percentage of lithium was 97.83% from the solution with 1.39 g.L-1 Li and 68.97 g.L-1 Na using the extraction system containing 0.4 mol.L-1 PHPD and 0.2 mol.L-1 Cyanex923 at the A/O ratio of 1:1, initial aqueous pH of 13.06, and 293.15 K, and the separation factor of lithium over sodium reached 475.06. On the basis of the McCabe-Thiele diagram of lithium extraction, two stages need to be used to achieve the complete extraction of lithium with an A/O ratio of 1.8. The extracted lithium species obtained by the slope analysis method were LiES and LiES2, where E and S represent the deprotonated PHPD and Cyanex923, respectively. The electrospray ionization mass spectra (ESI-MS) further confirmed that another two main Li adducts existing in the organic phase were LiE and LiE(H2O). The extracted lithium could be completely stripped from the loaded organic solution with hydrochloride of 0.5 mol.L-1 at an A/O ratio of 1:1. All of these will provide a theoretical basis for lithium separation from the mother liquor obtained during the process of lithium carbonate production.