Abstract: The conventional analysis of waterhammer and dynamics response of structure in piping system is divided into two parts, and the interaction between them is neglected. In this paper, the mechanism of fluid structure interaction under the double end break pipe in piping system is analyzed. Using the characteristics method, the numerical simulation of waterhammer structure interaction in piping system is completed based on 14 parameters and 14 partial differential equations of fluid piping cell. The calculated results for a loss of coolant accident(LOCA) in primary loop of pressurized water reactor show that the waveform and values of pressure and force with time in piping system are different from that of non interaction between waterhammer and structure in piping system, and the former is less than the later.