The invention discloses a method and system for recovering lithium from a waste lithium iron phosphate battery. The method comprises the following steps that a positive plate is disassembled from thewaste lithium iron phosphate battery; a binder in the positive plate is removed, valuable metal elements in the positive plate are leached by an acid solution, and an acidified leaching solution is obtained; ultrafiltration treatment is carried out on the acidified leachate by using an ultrafiltration membrane; by using a nanofiltration membrane technology, lithium ions in the acidified leachate are separated from other cations different from lithium ions, a solution containing lithium and a solution containing other cations are obtained, and then a reverse osmosis technology is adopted for concentration and enrichment correspondingly, wherein the other cations comprise iron ions; and a lithium precipitant is adopted to precipitate and separate lithium ions in the lithium-containing solution, alkaline substances are adopted to precipitate and separate iron ions in the solution containing other cations, and lithium recovery is realized. According to the method and system, the ultrafiltration-nanofiltration-reverse-osmosis combined technology is adopted, and the method and system have the beneficial effects of simple process, environmental protection, less acid and alkali consumption, good and stable membrane separation effect and the like.