Abstract: Based on the elaborate force and moment analysis with characteristics method and control volume integrating method for the piping system of primary loop under pressurized water reactor' loss of coolant accident(LOCA) conditions, the nonliner dynamic response of this system is calculated by the updated Lagrangian formulation(ADINA code). The piping system and virtual underpinning are specially processed, the move displacement of the broken pipe with time is accurately acquired, which is very important and useful for the design of piping system and virtual underpinning.