The invention discloses a method for extracting high-value 
alkali metal from 
salt lake brine or 
seawater through membrane extraction-back extraction. The method is implemented through 
continuous operation and comprises the following steps of: fixing an 
ion exchange blend membrane in a membrane component, allowing an organic solution containing an extracting agent to contact 
salt lake brine or 
seawater which contains 
alkali metal ions by a first 
ion exchange membrane, and allowing 
alkali metal ions to pass through the 
ion exchange membrane and be combined with the organic solution containing the extracting agent to obtain 
metal complex; then transmitting an organic solution of the 
metal complex to a second 
ion exchange membrane and allowing the organic solution of the 
metal complex to contact a back extraction solution by the second 
ion exchange membrane, and allowing the metal ions to pass through the 
ion exchange membrane to enter the back extraction solution; during membrane extraction-back extraction, and circulating feed liquid, the back extraction solution and the organic solution containing the extracting agent on one side of the first ion exchange membrane, on one side of the second ion exchange membrane and between the first and second 
ion exchange membranes; and performing back extraction until a certain concentration of the back extraction solution is reached, and separating 
lithium, 
rubidium or 
caesium precipitates to obtain the final product. The invention provides a high-efficiency, low-cost and feasible 
route for industrial production of alkali 
metal salts.