In the calculation of the traditional large fluctuation transition process, the influence of the connecting pipe at the bottom of the surge tank is often considered as the impedance hole. When the connecting pipe is long, the elasticity of the connecting pipe is neglected, which is liable to cause a large error. The surge joint have been mathematically modelized in two ways: with and without its connection to the pipe. Then these models have been used with the Gardel’s formula to treat the impulse coefficient of the surge tank. This study have been combined with concrete examples in order to figure out the influence of the connecting pipe’s diameter and length on the surge tank pressure, inlet pressure and unit speed of the surge tank. If the length of the linking pipe is too long, it would be necessary to consider its role. If the area of the linking pipe is too small, the surge tank will not be able to operate properly, causing water hammer waves.