The principle of distributed optical fiber sensing system based on Brillouin echo is analyzed, the transient coupled wave equation of Stimulated Brillouin Scattering (SBS) is deduced by a time domain perturbation method, and the transient solution of the SBS process is obtained. The Brillouin gain curves of the SBS process in Brillouin Optical Time Domain Analysis (BOTDA) systems using different pulse modulation are studied by Matlab simulation, and the 3-dimensional Brillouin gain spectra of the sensing systems are analyzed. The simulation results show that high Brillouin gain and narrow Brillouin linewidth as well as high spatial resolution can be achieved in BOTDA systems with bright pulse modulation, dark pulse modulation, and π-phase pulse modulation. And the largest Brillouin gain can be achieved in the π-phase modulation system.