The present invention provides a method for the treatment of mercury present in exhaust gas wherein, after a chlorinating agent is added to exhaust gas containing nitrogen oxides, sulfur oxides and mercury, the exhaust gas is subjected to a reductive denitration treatment in the presence of a solid catalyst and then to wet desulfurization using an alkaline absorbing fluid, the method being characterized by measuring the mercury concentration in the exhaust gas after the wet desulfurization; calculating a predicted value of the inlet mercury concentration before the reductive denitration treatment on the basis of the measured mercury concentration; and controlling the feed rate of the chlorinating agent added prior to the reductive denitration treatment, according to the deviation of the predicted value from a reference inlet mercury concentration, as well as a system for the treatment of exhaust gas. Thus, the present invention provides an exhaust gas treatment process permitting the removal of mercury, and this process makes it possible to achieve efficient operation of the system and maintain the performance thereof without exerting any adverse influence the units within the system.