In the system of Brillouin optical-fiber time-domain analysis (BOTDA), stimulated Brillouin scattering signal which produced by the interaction of pulse pump and DC continuous wave (CW )probe light should be detected. According to the frequency difference between two optical waves, BOTDA systems can be made as two types. One is the gain-mechanism, whose frequency of the pump pulse light is higher than the CW probe, and the other is the loss-mechanism, whose frequency of the pump pulse light is lower than the CW probe. In the gain-mechanism BOTDA, the sensing distance and the signal-to-noise ratio (SNR) are limited for the loss of pulse pump by the Brillouin interaction. The loss-mechanism BOTDA is studied by comparing the gain-mode BOTDA. The scheme of 100 km′s loss-mechanism BOTDA is set as the sensing system. It is found that the loss-mechanism BOTDA has better sensing distance and detecting performance than the gain-mechanism, and the worst SNR of the loss-mechanism is above 10 dB larger than the gain gain-mechanism under the same distance. So, the loss-mechanism BOTDA is a better choice for the long-range distributed optical-fiber sensing.