Abstract: The length of field pipeline varies in different battlefields. This paper, applying hydromechanics, probability theory and combination mathematics, builds a mathematics model calculating the change of pipeline capacity with the length of pipelines under the effect of reliability of pipes, connections, valves and pump stations, the time of consistent stable operation and frequency of adjustment of the system. Result calculated by the mathematics model indicates that pipeline capacity decreases as the length of pipeline increases and reliability of pump unit reduces. The mathematics model can help system planner to predict the maximum throughput of a pipeline to be built.