Water flow characteristics and distribution of polluted belt of chemical oxygen demand(COD) at estuary of Fenhe River into the Yellow River were simulated to discuss its migration and transformation law. According to the measured elevation values of section 59-60, interpolation method was used to supplement the terrain data and unstructured grid was used to divide the terrain. A two-dimensional hydrodynamic and water quality model was established, and alternating direction implicit method(ADI) was adopted to give an integral of the flow's continuity and momentum conservation equation. The result showed a high match between the simulation and the actual measurement. In addition, the velocity distribution and contaminant concentration distribution were given by simulation of COD in 50%, 75%, 90% hydrologic year types in the estuary of Fenhe River into the Yellow River. Moreover, as the increase of hydrologic frequency years, the area of river mouth beach formed was reduced, and the COD value reached the standard over the distance of 2.35, 3.00, 4.20 km.