Prefractionation system of pygas hydrogenation plant mainly includes depentanizer and deoctanizer systems. Some problems are often encountered under abnormal conditions, such as the overhigh C₄ light hydrocarbon content of the feedstock of crude pyags during start-up or in summer, the decomposition of DCPD in deoctanizer bottom at higher temperatures, and the polymerizaiton coking and blockage of easily-polymerized components at higher temperatures. These problems will cause operation fluctuation of the towers, affect product quality, and even lead to overpressure or affect the long-term operation of the towers. The depentanizer and octanizer under abnormal conditions are simulated and calculated respectively. Analysis is carried out combined with the actual production and the simulation results. The design of depentanizer and octanizer is optimized respectively according to the analysis results.