The invention discloses a low-viscosity water-based ultraviolet (UV) ink. The low-viscosity water-based UV ink consists of the following components in percentage by mass: 30-50% of a water-based acrylic prepolymer, 20%-50% of a water-based reactive diluent, 10%-30% of a solvent, 1%-4% of a photoinitiator, 5%-10% of a pigment and 2%-5% of an assistant. Styrene used in a preparation process of the water-based UV ink disclosed by the invention can effectively improve mechanical strength of an ink film, and acrylic acid and hydroxyethyl methylacrylate (HEMA) are beneficial to improving water solubility of the ink. Meanwhile, the hydroxyethyl methylacrylate has double bonds and aldehyde groups at the same time, so that the light curing rate of the ink is greatly improved.