Aimed at the influence of emergencies on the reserved water for government emergency so as to appear the features of bing uncertain, short and periodic,the paper applied the case reasoning method to the forecast of reserved water for governmental emergency.It introduced dummy variables to describe the qualitative factors and reduced attribute based on rough set theory. Based on the water demand of historical emergency of e similar cases and the weight of time series, the paper forecasted the reserved water demand for government emergency. Finally, it applied the method to the emergency of river pollution accident resulted from the leakage of hazardous chemical and illustrated its practical value.