The Smith′s model and Küng′s model for calculating the threshold of stimulated Brillouin scattering (SBS) are analyzed and discussed. The more exact method is investigated. The relationship between critical gain coefficient and fiber length is obtained, which shows that the critical gain coefficient can be considered as constant only when fiber length is long enough. The SBS threshold of 25 km single mode fiber is measured by experiment. Finally, the experiment to measure SBS threshold using Brillouin optical-time-domain reflectometer (BOTDR) is done. The results are well agreed with the theoretical predication.