Activated carbon is an economic material to grab free chlorine in drinking water to reduce potential health risks. In this research, chemical kinetic reaction of the activated carbon for free chlorine removal was studied, which exhibited the first order kinetic reaction performance. A relationship between the rate of chlorinated water flowing through the activated carbon, and the free chlorine (ClO−) concentrations before and after the reduction by activated carbon was obtained. The logarithm of the free chlorine concentration (lnC) was linearly related to the reciprocal of the volume flow rate (1/v). The slope was dependant on the kinetic constant of the activated carbon dechlorination reaction. This research is beneficial for the scientists and engineers to study the mechanism of the chemical kinetic reaction of the free chlorine removal by activated carbon and design activated carbon-loaded water purification reactors.