For the standard single-mode optical fiber, the frequency shift of Brillouin scattering light is the function of the temperature or strain. The temperature or strain of the optical fiber can be obtained when Brillouin frequency shift is able to be detected. Long-distance sensing Brillouin scattering distributed optical fiber sensor (DOFS) has two structures of optical time-domain analyzer (OTDA) and optical time-domain reflectometer (OTDR). Although Brillouin OTDR only process single end of the optical fiber, it suffers short sensing distance because of weak spontaneous Brillouin scattering. Brillouin OTDA possesses the merit of long-distance sensing for the relatively strong stimulated Brillouin scattering. One key point of achieving Brillouin OTDA is to produce the frequency-shifted probe light. Here, an optical fiber laser with linewidth less than 1 MHz is employed as the single optical source, a microwave electro-optical modulator is used to produce the frequency-shifted probe light, and a method of orthogonal polarization control is adopted to reduce the effect of the optical-polarization-related problem. A laboratorial system of 50 km distributed optical sensor is achieved with 2 ℃ in temperature resolution, and 10 m in spatial resolution.