The invention relates to a preparation method of a solvent-based PU flocking glue. The method comprises the following steps: (a) dissolving polyester diol, polyester triol and thermoplastic TPU in a mixed solvent, and performing dehydration to obtain a first mixture, wherein a mass ratio of the polyester diol to the polyester triol to the thermoplastic TPU is 100:(40-60):(15-25); and (b) adding liquefied MDI to the first mixture, sealing the obtained mixture, continuously mixing the sealed mixture by a shaker for 10-15 h, performing sealed standing for 3 d, and adjusting the solid content of the obtained mixture to 38-43% and the viscosity to 200-400 cps. The solvent-based PU flocking glue can be directly used in the flocking process of PVC sealing strips without on-site preparation, the curing conditions of the solvent-based PU flocking glue meet the requirements of the online flocking process (120 DEG C, 3-5 min), and the solvent-based PU flocking glue meets the flocking wear resistance requirements.