Abstract Recently, once the amounts of precipitation are abnormal at everywhere in Taiwan, it either brings up disaster of floods, nor droughts. The main water supply source at the north of Taiwan– Shihmen Reservoir, is facing the major problem of water shortage. To let a dam work properly, first, it depends on the enough amount of precipitation; secondly, the right way of releasing water is also very important. Since “Shihmen Reservoir” had been built in 1964, it started to use the M-5 curve to rule the dam. Due to the growing up of economy, industry, and the quality of lives, the needs of water has also increased. Therefore, it is important to enforce the operation and management of reservoir to increase the use rate of water resources. So, this study used the historical discharge informations,the amount of water using for each goal with the operation rule of the reservoir to simulating the operation of the reservoir. At the same time,the M-5 curve foe the reservoir were analyzed and compared each other. Accommodate of M-5 curve in Shihmen Reservoir, using the comparisons of each index, to figure out any possible amendments to approve the problem of shortage of water. Also, under the condition of invariant water right shared the amount of storing water increased was discussed due to the raised upper level position of water. Though this study, we will increase up 240m higher the minimum of upper level water, and re-imitate the history data of water behave. As the result, it surely can make great amount of water pondage, and decreasing the time period of water defection. However, water increasing is definitely affecting the protective power and the structure of the dam itself, and also the safety of the residents live around downriver. Although, flood-controlling function has been calculated safely within a certain arrangement, this is more important to build more new useful establishments to protect our land before any happened.