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Accurate cooling crystallization control has a significant impact on terminal crystal product quality. Herein, a hollow fiber membrane-assisted cooling crystallization (MACC) was investigated to achieve nucleation induction and self-seeding operation. Poly(tetrafluoroethylene) (PTFE) and polyethersulfone (PES) membranes with thermal conduction properties were introduced to accelerate the nucleation at a supercooled membrane surface, and the detached crystal seeds were then transferred into the crystallizer. A polymeric membrane can achieve nucleation control and hinder the potential secondary nucleation, which has been validated by online detection experiments. This advantage rendered excellent MACC performance in the manufacture of high-purity thiourea crystals, which possessed the desired morphology, a large mean size (>1.35 mm), high purity (>99.5 wt %), and narrower size distribution.