The effect of catalyst activity on a 900 000 ton y(-1) gasoline diesel hydrotreating unit is analyzed on the basis of the integration of hydrotreating reactor and the heat exchanger network (HEN). The reactor's inlet/outlet temperatures versus catalyst activity relation is identified on the basis of simulation and coupled with the composite curve. On the basis of this, a graphical method is proposed to integrate the hydrotreating reactor and the HEN, and analyze the effect of catalyst deactivation on the energy consumption of HEN. Three matching schemes are proposed for the HEN, and the variations of the heat exchange area along the catalyst deactivation are analyzed. The optimal matching scheme is identified with both operating cost and capital cost considered. For the studied gasoline diesel hydrotreating unit, catalyst deactivation has a significant influence on the energy consumption, and the minimum heating utility consumption decreases from 2680 kW to 0 as the catalyst activity decreases from 0.9 to 0.44. The study shows that the heat exchange area of a heat exchanger changes significantly along catalyst activity, and its maximum area might appear at neither the highest nor the lowest catalyst activity. The proposed integration method can be applied to analyze the effect of catalyst deactivation efficiently.