Abstract: Mathematical deductions proved that the pollutant concentration at location P(x, y) in the bank discharge situation is equal to the one at location Q(x/4,±y/2) in the central discharge (the origin of both is at the discharge point). According to our numerical experiments and curve regression of the results, this paper gives the parameters of the pollutant mixing zone including the coordinates of the maximum length and width and its area, the practical formula for concentration distribution along the river's center line and bank, and the approximate outer boundary curve equation of the pollution mixing zone. The results have great practical value in the calculation of the pollution mixing zone in medium-width rivers and the estimation of the concentration at environmentally sensitive locations. It also provides an accurate, convenient, fast and practical method for engineers.