Provided are a layer-separation method of a spent caustic solution, and a recycling method of an additive, and more particularly, a layer-separation method of a spent caustic solution including: injecting an additive and an acidic compound to the spent caustic solution occurring from a refinery process to break down a red oil emulsion and to perform layer-separation into an upper layer fraction and a lower layer fraction, wherein the additive is an aliphatic hydrocarbon compound having a water solubility of 0.1 to 10 g/L at 20° C.