The authors researched the water-filling process of a pressured-pipeline in the long distance. According to the characteristics for the pressured  pipeline in the long distance during the filling process, the strategies of sectional treatment and water filling in succession are adopted. For each segment of the pipe, a mathematical model is established with adopting the sluice gate outflow type of the broad crest weir and using the Clapeyron equation according to the filling principle under the low flow rate in combination with the characteristics of exhaust gas of an air valve. On the basis of the different water filling discharges and the different air valve intervals, authors calculate the pressure changes on the feature points in the pipe. The calculated results indicate that: the greater the water filling discharge, the longer the air valve interval ； the greater the feature points pressure, the higher pipe risk. This computation results can provide safety assessment foundations for the pressured-pipeline filling process in the long distance.