Abstract: The reaction rule on production of light aromatics (BTXE) of benzene, toluene, xylene and ethylbenzene from hydrotreated LCO by FCC process is studied on molecular level. The results show that the content of heavy monocylic aromatics and FCC reaction conditions are the key factors which influences the yield of BTXE and that the optimal reaction conditions for converting LCO to light aromatics are: moderate hydrotreating severity (hydrogen content 11.0% in hydrotreated LCO), high reaction temperature (over 550 ℃) and middle ratio of catalyst to oil (more than 8) of FCC process. In the range of experimental conditions, the proportion of the apparent cracking reaction of monocyclic aromatic hydrocarbons in hydrotreated LCO is above 73.4%, the condensation reaction is less than 14.7%, and the rest is unconverted monocyclic aromatics. Increasing FCC reaction temperature is favorable for monocyclic aromatics cracking. The once-through yield of light aromatics reaches 14.3% under experimental conditions, about 1/3 of all monocyclic aromatic hydrocarbons in FCC products. It is concluded that the integration of hydrogenation and FCC process is a cost-effective technology to convert inferior LCO to high value BTXE light aromatics.