Abstract: Chromium ion was separated from water by using the ferrous coprecipitation foam separation method. The influences of pH value, the molar ratio of Fe²⁺ to Cr（Ⅵ）, the concentration of a linear alkyl benzene sulfonate(LAS) and gas liquid ratio were investigated.When chromium ion concentration was 8 mg·L^-1，the coefficient of removal was 97.1%.According to the similar physical behavior of foam separation process and chemical reaction process,the equivalent chemical reaction constant was introduced,and the macroscopical dynamics of using coprecipitation foam separation to remove chromium ion from water was studied.The result indicated that the process of coprecipitation foam separation could be regarded as a first order reaction.The mathmatical model of this process combined with the determination of residence time distribution was analysed and discussed.