For water-supply engineering, air vessel is often used as an effective protection measure of water hammer. It is usually arranged behind pump nearby. Under the circumstance of ensuring the safety of water conveyance system, the volume of air vessel should be reduced as far as possible. Then the economy of investment and convenience of management can be met. An effective protection measure to optimize the size of the air vessel, setting air vessel behind pump and bidirectional tower at the end of pipeline, is discussed in this paper. Its protection principle and feasibility are also analyzed. Compared with the program of only setting an air vessel behind pump, the joint protection program can significantly reduce the volume of air vessel when protective effects are roughly the same.