Soot is an undesired co-product during thermal-chemical disposal (incineration, pyrolysis, etc.) of plastic wastes at high temperatures, while also regarded as a valuable nanoscale carbon-based material if with proper production and post-treatment methods. In this paper, the pyrolysis of polyurethane (PU) plastics, a major composition of plastic waste, was conducted in a fixed-bed reactor to investigate the soot formation mechanism in depth. The effects of pyrolysis temperature (1000-1300 degrees C) and volatile residence time (0.2-2 s) on the yield, micro-morphology, composition and reactivity of soot were studied. Results show that the initialization and growth of soot particles during PU pyrolysis requires a certain high temperature and long volatile residence time. With a volatile residence time of 2 s, the soot yield is 11.0 wt.% at 1000 degrees C and increases to 24.5 wt.% with the pyrolysis temperature increasing to 1300 degrees C; while when the volatile residence time decreases to 0.2 s, the soot cannot be observed until 1200 degrees C. When the pyrolysis temperature increases 1000-1100 degrees C, the X-ray diffraction (XRD) patterns indicates an enhanced graphitization thereby a lower reactivity of soot oxidation, which is approved by onion-like layered structures initially observed by the high-resolution transmission electron microscope (HR-TEM) at 1100 degrees C and 2 s. When the pyrolysis temperature increases from 1100 degrees C to 1200-1300 degrees C, NaCI is observed in soot particles, which catalysis promotes the reactivity of soot oxidation. Gas chromatography and mass spectrometer (GC-MS) was used to measure the gas and tar compositions, especially the polycyclic aromatic hydrocarbons (PAHs), are proved essential for the formation of soot precursor. A large number of PAHs with ring number 4 start to form at 1100-1200 degrees C. Based on the soot characterization and gas-tar compositions, a pathway from light fragments and mono-ring aromatics to heavier aromatic compounds is proposed aiming to explain the chemical evolution of soot precursor particles.