The present invention discloses a method for producing lithium carbonate from a low-lithium brine by separating magnesium and enriching lithium. A salt-lake brine is used as a raw material and is converted into halide salts through dehydration by evaporation and separation by crystallization; the halide salts are directly extracted using trialkyl phosphate or a mixture of trialkyl phosphate and monohydric alcohol, and an organic extraction phase as well as remaining halide salts are obtained after solid-liquid separation; reverse extraction is performed on the organic extraction phase to obtain a lithium-rich solution with a low magnesium-to-lithium ratio, and lithium carbonate is obtained after concentration and removal of magnesium by alkalization. The used solid-liquid extraction method is simple with no co-extraction agent used, and a solute distribution driving force is strong, unaffected by phase equilibrium of the brine extraction agent. The mass ratio of magnesium-to-lithium significantly decreases in the extraction phase.