To address the issues of concealed thermal release substances during the pre-smoldering stage of materials and the delayed response of traditional infrared and smoke sensors， a novel early detection method for the pyrolysis of electrical insulation materials based on cloud chamber nucleation is proposed. By rapidly expanding gas within a sealed chamber，pyrolytic particles and 1~10 nm volatile precursors released in the early pyrolysis stage are converted into detectable condensation nuclei from 0.5 μm to 10 μm under supersaturated conditions. Particle concentration is then quantitatively measured using laser right-angle scattering. An integrated high-time-resolution detection system with a single measurement cycle as short as 8 seconds was developed and validated on polyvinyl chloride (PVC) cables and silicone rubber insulators. Results show that significant particle signals can be detected even before visible smoke appears and well below ignition temperature， achieving peak concentrations of 2.543 2 million particles·cm -3 for a 10 cm PVC cable and 2.419 7 million particles·cm -3 for a 90° sector sample of an insulator. The method can trigger alarms within 20~30 seconds after the onset of pyrolysis， offering higher sensitivity and earlier response than infrared and smoke detection methods. Moreover， it does not rely on a direct line-of-sight path， enabling effective identification of hidden thermal faults behind structural obstructions.