A method for impact analysis of a reservoir's prior release operations precedential to floods is developed as a fundamental study in this paper. A Monte Carlo simulation model of a reservoir's prior release operation coupled with artificial generation model of inflow predictions is proposed here. The inflow predictions can be generated with random errors based on given accuracies of the prediction so as to take impacts of inflow prediction's accuracy on the effectiveness of prior release operations into consideration. Impacts of prior release operations on flood mitigation and water storage for water utilization are then analyzed and discussed by use of proposed simulation model for assumed reservoir operations which are derived from an existing multi-purpose reservoir.