Abstract: The guided waves' propagation characteristics are studied in SFR (Sodium cooled Fast Reactor) pipes when the speed and temperature of liquid sodium are changed. We derivate the dispersive function of the guided waves in the condition of the liquid sodium is moving in pipes, and numerically calculate the dispersion curves and the time domain waveforms of the longitudinal modes of guided waves in different temperature and speed of liquid sodium. As results, the changing temperature hardly influents the first longitude mode, while it effects the higher order longitude modes, however, the dispersion curves move a little toward high frequency when the liquid sodium speed increasing, and this can be ignored in practical testing. By the simulation of received time domain waveforms, the further investigation of the influence on guided waves of the temperature and speed of liquid sodium is conducted, and the standard of choosing the modes and excited band of guided waves is given, which is suitable for ultrasonic non-destroyed testing and evaluating of SFR pipes by the comparison of the excited characteristics of different modes and the waveforms of guided waves excited by different frequency band.