The 2-D tide water quality model was adopted to simulate and forecast the impact of emergency water pollution accident occurred in Qiantang River on water quality. The result showed that the transport velocity of polluted water distinctly increased with the increase of runoff and the maximum concentration also decreased. In the same time, with the dispersion and dilution of pollution in the process of transport, the total duration increase and the maximum concentration decreased gradually along the river. The result of simulation and calculation revealed that the increase of discharged flow at the upstream of river plays a very important role in accelerating the polluted water to move downward and diluting the pollutant body so as to lower the pollutant concentration when the emergency water pollution accident occurred. The result can provide an important theoretical basis for the treatment of unexpected water pollution emergency.