Abstract: Based on one dimensional method of characteristics(MOC), a numerical model of pressure wave progation is presented in the paper. A special code is programmed to analyze and calculate waterhammer resulted from rapid opening or closing of valve in the experimental sodium circus of fast breeder reactor(FBR). In the model, a new outer boundary condition, sodium cavity is included. Model of bubble's discrete distribution is adopted to simulate generation and collapse of the bubble with the pressure's decreasing and increasing. The results demonstrate that the model of pressure wave progation is valid.