The invention discloses an extraction system for extracting alkaline metals or alkaline-earth metals. The extraction system comprises an extraction agent and a synergistic extraction agent, wherein the extraction agent is a diketone compound, and the synergistic extraction agent is a neutral phosphoric-oxygenic compound. The invention further discloses a method for extracting lithium ions from an alkaline metal solution and extracting alkaline-earth metals from a mixed solution of alkaline metals and alkaline-earth metals through the extraction system. The extraction system has excellent effects for extracting and separating alkaline metals or alkaline-earth metals; and meanwhile, a synergistic extraction agent, ferric trichloride, in the prior art is avoided in the extraction system, so that problems such as high acidity caused by the ferric trichloride and the like can be avoided, the acid dosage and the cost can be reduced, and the corrosion to devices can be reduced as well. After lithium ions or alkaline-earth metal irons are extracted by using the extraction system, the reverse extraction and regeneration processes are simple, and the operation is facilitated.