The invention belongs to the field of non-ferrous metal wet metallurgy, and particularly provides an application method of trialkyl naphthalene sulfonic acid. The trialkyl naphthalene sulfonic acid isadded into dialkyl naphthalene sulfonic acid and pyridine carboxylic acid so as to accelerate an extraction and phase separation speed. In addition, the invention further discloses a synergistic extraction agent containing the trialkyl naphthalene sulfonic acid. The synergistic extraction agent is prepared from the trialkyl naphthalene sulfonic acid, the dialkyl naphthalene sulfonic acid and thepyridine carboxylic acid. The invention further comprises preparation of the synergistic extraction agent, and application of the synergistic extraction agent in directly extracting nickel-cobalt froma multi-metal acid solution containing Ni2+, Co2+, Fe3+, Ca2+, Mg2+ and the like at the same time. The long-carbon-chain trialkyl naphthalene sulfonic acid is added into the synergistic extraction agent containing the dialkyl naphthalene sulfonic acid and pyridine carboxylic acid, so that the extraction and phase separation speed of the extraction agent can be remarkably improved, the extractionand clarification speed meets the requirement on an industrial design of a mixer-settler, the extraction rate of the nickel-cobalt directly extracted from the multi-metal acid solution containing Ni2+, Co2+, Fe3+, Ca2+, Mg2+ and the like at the same time cannot be influenced, and a removal effect on impurities such as Fe3+, Ca2+, Mg2+ cannot be influenced.