The invention discloses a zinc-nickel alloy blue passivator and a preparation method thereof. The passivator comprises an agent A and an agent B, wherein the agent A comprises a trivalent chromium compound, oxalic acid, citric acid, trisodium phosphate, sodium hydroxide, a complexing agent and deionized water; the agent B comprises cobalt salt, nickel salt, sodium salt, a stabilizer and deionizedwater. The preparation method of the zinc-nickel alloy blue passivator comprises the following steps: mixing the trivalent chromium compound, oxalic acid, citric acid and trisodium phosphate with water, heating and preserving heat to obtain a trivalent chromium compound solution, dropwise adding a sodium hydroxide solution into the trivalent chromium compound solution to obtain the agent A, mixingcobalt salt, nickel salt, sodium salt and the stabilizer with water to obtain the agent B, and mixing the agent A and the agent B with water to obtain the zinc-nickel alloy blue passivator. The bluepassivator is capable of reducing loss of coating; the service life of the passivator is longer; meanwhile, the passivated appearance is better; the corrosion resistance of the coating is greatly improved.