Abstract: This paper focused on the reasons for deterioration of slurry quality in the heavy oil catalytic pyrolysis process, which leads to coking in fractionator, unqualified products, lower ethylene and propylene yields and the oil slurry system plugging, threatening the long term stable operation of the unit. By the operation data analysis and the catalyst characterization by NH3-TPD and BET, considering the reaction mechanism, it is concluded that high level of Fe in the feedstocks was the main cause of the above issues. The measures are suggested: substantial replacement of balance agent and iron removal from feedstocks.

Key words: Catalytic Pyrolysis, slurry quality, catalyst activity, Fe content in feed, Iron removal agent