Lateral and vertical diffusion coefficients of open channel is one of important water quality parameters to measure the mixedly transporting capacity of pollutants near the bank flow. Whether the value is accurate or not is directly related to the reliability of the water quality forecast in open channel flow. Based on the theoretical calculation method of pollutant mixing zone, the unified equation of the standard curve and curved surface outside the borders of pollution mixing zone is deduced, including the characteristic parameters of the maximum length Ls, maximum width bs and maximum depth ds et al. The shape of the curve approximates to a semi-ellipse, and the shape of curved surface is akin to a part of ellipsoid, which shows the pollution of the mixing zone has similarity. The calculation equation for determining the lateral or vertical diffusion coefficient by the maximum length, maximum width or maximum depth of the outer boundary of pollutant mixing zone and the mean velocity is given. The calculation method of the lateral or vertical diffusion coefficient which is overall controlled by the area or volume of pollutant mixing zone, and the practical method of vertical diffusion coefficient which is determined by the concentration of surface transverse integration, are proposed. By analyzing the data of field observation, the lateral diffusion coefficient of the downstream of Guangfu River in low water season is 0.27 m2/s.