The invention relates to a water-based metal ink for UV offset ink and a preparation method thereof. The water-based metal ink comprises the following components by weight: 25-35 parts of water soluble resin, 30-50 parts of a solvent, 1-5 parts of a photoinitiator, 10-30 parts of a metallic pigment, 2-5 parts of a fastness assistant, 2-4 parts of a leveling assistant and 1-3 parts of a wetting and dispersing assistant. Specifically, the solvent is a mixed solvent of deionized water and ethanol in a mass ratio of 1:1, the metallic pigment is bronze powder or aluminum silver powder. The preparation method includes: firstly fully mixing the metallic pigment with the solvent evenly, then adding water soluble resin, the photoinitiator and assistants in order, stirring the materials fully till uniform dispersion, then carrying out filling, sealing and packaging, and performing aluminum foil sealing, thus obtaining the water-based metal ink finished product. Compared with the prior art, the water-based metal ink provided by the invention is green and environment-friendly, has the advantages of good binding fastness with UV offset ink, difficult fall, simple preparation process, good finished product stability, and production cost saving, and is suitable for industrial scale and high efficiency production.