The quick buildup of higher pressure drop in the fixed‐bed reactor of a hydrotreating unit of a petroleum process is identified and improved. The results indicated that coke deposits formed in the catalyst bed are due to both the polymerization initiated by the dissolved oxygen in the presence of olefins, and the dehydrogenation reaction formed by hydrogen deficiency in the event of power failure in the unit. The buildup of pressure drop in the hydrotreating unit is improved by adding both an oxygen stripper in the feed stream to minimize the dissolved oxygen and catalysts with high void fraction and low activity in the top of the reactor to increase the tolerance of coke deposits. After revamping, the hydrotreating unit can operate at design throughput and the operation cycle time is not limited by the pressure drop. The results of this study are a breakthrough in the study of hydrotreating units. A correlation method was also developed to check the operation cycle time of the hydrotreating unit.