The invention belongs to the technical field of production of liquid resin, and particularly relates to a preparation method of alpha-methyl styrene liquid resin. The problems of high raw material cost, high reaction temperature, poor low-temperature resistance and poor viscosity and chromaticity of tackifying resin for a hot melt pressure sensitive adhesive in the prior art are solved. The preparation method of the alpha-methyl styrene liquid resin comprises the following steps: S1, stirring and mixing alpha-methyl styrene, short-chain olefin and a reaction solvent, then adding the mixture into a reaction kettle, and adding a strong acid catalyst and an antioxidant for reaction; S2, adding alkali liquor and a demulsifier to remove the strong acid catalyst, and carrying out vacuum distillation to obtain a distillate; and S3, fully mixing the distillate with polyethylene glycol to obtain the final product. The preparation method provided by the invention is simple, the reaction condition is mild, the obtained liquid resin is safe and environment-friendly, the oxidation resistance is good, the low-temperature resistance is excellent, the liquid resin can be normally used in cold winters, the viscosity and the chromaticity are good, and the application range is wide.