Abstract: In order to solve the insufficiency in the traditional definition for SBS threshold, a new definition and a noval measuring technique are proposed. The transmission of beam with different energy was measured at different lengths of water, and the attenuation coefficient of laser beam with different pulse energy was tested. The SBS threshold under different conditions was confirmed based on the attenuation coefficient. It was found during the experiment that the SBS threshold under the ideal stable state and the ideal transient state is a constant, it is related only to the pump laser and medium attribute, but not to the medium length. To other cases, the relationship between SBS threshold and length will be studied through further experiment.