Abstract: Formation reasons of fouling of the rapid cooling heat exchanger for ethylene plant were analyzed by methods of powder XRD combined with SEM-EDS, FTIR, and elemental analysis of carbon and hydrogen. The results show that the fouling of rapid cooling heat exchanger were slowly formed from reaction of carbon dioxide in hydrocarbon pyrolysis gas and trace calcium ion in cracking material to produce calcite, iron’s electrochemical corrosion products of magnetite, maghemite and hematite crystals, and hydrocarbon cracking reaction’s condensation product of amorphous carbon. The application shows that powder X-ray diffraction method combined with artificial intelligence can quickly and accurately identify crystal phase in the fouling of rapid cooling heat exchanger for ethylene plant.