Abstract: In some OCT-M FCC naphtha hydrodesulfurization units, rapid rise of reactor pressure drop happened frequently, cause analysis was conducted, such as analyzing the compositions of coke sample from the reactor top part and feedstock as well. It is concluded that the accumulation of amorphous coke deposits on the top part of reactor is the main cause of building up high pressure drop, especially when OCT-M unit processing purchased FCC naphtha and/or FCC unit processing high aromatics blending feedstock. To solve the problem, when processing long-stored purchased naphtha, the naphtha should be pretreated by fractionation column of FCC unit to remove impurities, such as gum. Heavy aromatics feed is not suitable to be blended into FCC feedstock, it will increase the gum content and end point of FCC naphtha significantly, blending should be avoided.