By pooling or slowing flow behind a dam, there are temperature changes both within the reservoir and downstream. Analysis of these changes have important significance in estimating effects of hydroelectric powers on the local climate, aquatics and crops. Based on long-term monitoring data, stratified temperature patterns occurred in Longyangxia reservoir on the upper sections of the Yellow River in Qinghai province is analyzed. Because of important impacts of air temperature on the top layer water temperature of a reservoir and a river, a new model is established to assess net impacts of Longyangxia reservoir on the water temperature within the reservoir and downstream of the dam. In comparison with conventional models, the new model adds correctional factor for relations of the air and water temperature and avoids impact of natural factors on changes of the water temperature, thus, resulting in exact calculating results.