The water quality health risk assessment model is usually applied to the evaluation of the drinking water quality and sudden water pollution accidents. In the course of establishing the index system of transboundary paroxysmal water pollution accidents,for the non-steady state features of characteristic pollutants concentration,the traditional water quality health risk assessment model of EPA was modified as to be a significant index of our forecasting system. On the basis of this,the water quality health risk of two typical transboundary water pollution accidents: the nitrobenzene pollution of Songhua River and the cadmium pollution of North River,was calculated by utilizing the historical data. The results show that the modified model can describe the potential harmful levels to human health quickly and quantitatively,which provides the scientific evidence for trans-boundary paroxysmal water pollution accidents forecasting and its emergency treatment.