In order to test the mechanical strength of optical fiber, the fiber tensile tester and special fixture are designed. They are used to test the mechanical strength of common temperature fiber and vulcanized fiber. In common temperature, the average maximum tensile strength of Corning 900 fiber is 61.4 N, and that of SMF-28 bare fiber is 49.3 N. After vulcanization, the average maximum tensile strength of both kinds of fibers is 57.3 N and 47.6 N respectively. By the method of fracture model analysis, the scanning electron microscope(SEM) is used to observe and detect the common temperature fiber and the vulcanized fiber. And the difference of the sections between the two fibers is compared. The X-ray photoelectron spectroscopy(XPS) is used to further detect. It is proved that sulfur exists in the vulcanized fiber. The reasons for the decline of mechanical strength of vulcanized fiber are explained.