The invention provides a full-automatic separator for expanded cut tobaccos in formula cut tobaccos and a control method thereof. The full-automatic separator comprises a cut tobacco separation device and an automatic control device thereof, wherein the separation device comprises a mixing container and a separation pipe which is located above the mixing container and is communicated with the mixing container through a ball valve, an infusion pump, a liquid storage tank, a cut tobacco collection box, an expanded cut tobacco collection box, a pipeline system and a ball valve part arranged in the pipeline system; the whole pipeline system forms a circulating sealed loop; the control device comprises an operation panel, a control system and liquid-level induction elements; the start and shut-off of the infusion pump and each connection valve are controlled by the control system; the liquid-level induction elements are arranged on the liquid storage tank and the mixing container respectively; and the induction elements are connected with the control system. The separator can be used for realizing a full-automatic separation process under a sealed condition, is accurate, rapid and stable and guarantees the consistency of each process in a plurality of times of separation; and manual errors are reduced, the accuracy of testing is improved, the labor intensity is alleviated and the automation degree of testing is improved.