The invention provides a synthetic method and application for sodium carboxymethyl sodium trithiocarbonate. The synthetic method comprises the following steps: adding carbon bisulfide into an aqueous solution of sodium sulfide, allowing the obtained mixed solution to completely react, then adding sodium chloroacetate into reactants, and after completion of reaction, obtaining a product containing sodium carboxymethyl sodium trithiocarbonate. The synthetic method provided by the embodiment in the inveniton is easy to realize; and the synthesized sodium carboxymethyl sodium trithiocarbonate is applied in flotation separation of lead-zinc sulfide ores, is non-toxic and highly-efficient, and has extensive industrial promotion values.