The invention discloses a quasi-two-dimensional mathematical model beach slot section division method based on a momentum correction coefficient. According to the method, the roughness of a riverbed is determined by adopting the bed sand particle size, the average flow velocity of compound section transverse nodes under different water level conditions is calculated by adopting a new resistance, and the change rule of a momentum correction coefficient along with the water level is calculated according to a momentum correction coefficient formula. The method comprises steps of numbering the calculated momentum correction coefficients from small to large along the water level; and calculating a difference value between two adjacent momentum correction coefficients, and considering that the area can be divided into a flood bank when the cumulative amplitude variation of the momentum correction coefficients is greater than 0.02 under the same change trend, and the elevation range of the flood bank is the first water level and the last water level of the momentum correction coefficients. The method can quantitatively identify the main channel and the flood bank of the compound section,and is simple in calculation, high in operability and high in accuracy.