In the last decades, Taiwan has more frequently suffered from extreme floods than before. In coping with the aggravating flood threaten, this study proposed a decision analysis procedure to assist reservoir pre-release operation in advance of typhoon. It consists of (1) water supply simulations for quantifying the shortage risks under different initial storages in the beginning of different ten-days, (2) uncertainty evaluation of typhoon rainfall with forecast information, (3) calculation of reservoir inflow during flood, (4) simulations of shortage risks under different pre-releasing volumes. Upon assessing the shortage risks by different pre-release decisions, the decision maker can then select a pre-release strategy based on his or her acceptable incremental shortage conditions. The operations of Tsengwen Reservoir at southern Taiwan during typhoon Jangmi in 2008 is presented to illustrate the significant contribution of pre-release on mitigating flood potential.