The invention provides a method for calculating the assimilative capacity of a water functional area in a freeze-up period, and aims at providing a method for calculating the assimilative capacity of the water functional area suitable for the cold region river freeze-up period. The method is characterized by including the steps of establishing a one-dimensional water quality model of the assimilative capacity of the water functional area and the like. The method has significance in analyzing and regulating the water environment risk of the cold region freeze-up period.