The invention relates to the technical field of circuit boards, in particular to a composite activating agent for electroless nickel/immersion gold, a preparation method of the composite activating agent, and an activating method. The composite activating agent is prepared from the following raw materials in parts by weight: 1030-1150 parts of a solvent, 0.7-2.9 parts of a surfactant, 0.15-2.7 parts of palladium acetate, 0.02-0.05 part of copper sulfate, and 2.7-4.5 parts of a synergist. The preparation method comprises the following steps: uniformly mixing acetic acid, palladium acetate, 2-chloroethyl pyridine and 2-amino-5-nitrothiazole in proportion to obtain a mixture A, and then uniformly mixing the mixture A, a solvent, a surfactant and copper sulfate in proportion to obtain the composite activating agent. The activating method comprises the following steps: immersing a pre-treated circuit board in the composite activating agent, then taking out the uniformly immersed circuit board, washing with water, and air-drying to obtain the uniformly activated circuit board. According to the invention, the activating agent has the effect of reducing skip plating and burrs of the circuit board.