The invention relates to the technical field of pressure-sensitive adhesive, particularly to a UV photocuring solvent-free ultrahigh viscous pressure-sensitive adhesive and a preparation method thereof. The pressure-sensitive adhesive consists of the following raw materials in parts by weight: 35-45 parts of butyl acrylate, 10-20 parts of acrylic acid 2-ethylhexyl, 0.5-2 parts of propenoic acid, 1-2 parts of hydroxyethyl acrylate, 10-30 parts of terpene-phenolic resin, 0.2-2 parts of benzoyl peroxide, 2-3 parts of dilauroyl peroxide, 0.1-10 parts of a photoinitiator, 0.1-5 parts of an anti-oxidant and 97-140 parts of a solvent carrier, wherein the solvent carrier is a mixture consisting of 60-70 parts of toluene, 32-50 parts of ethyl acetate and 5-20 parts of isopropyl alcohol. The pressure-sensitive adhesive not only has excellent compression sensitivity and weather resistance of acrylate, but also has the characteristics of terpene-phenolic resin of being high in softening point, light in color and luster, strong in adhesive strength, large in cohesive force, high in heat resistance, aging-resistant and nontoxic.