By using the theory of coherent detection, a Brillouin optical time domain reflectometer sensing system was proposed based on the external modulation of multi-longitudinal mode Fabry-Perot laser and the self-heterodyne detection of Rayleigh and Brillouin scattering. The principles of the stimulated Brillouin scattering threshold improvement by the utilization of the multi-longitudinal mode Fabry-Perot laser, the self-heterodyne detection of Rayleigh and Brillouin scattering were analyzed, the expression of signal-to-noise ratio of the system was deduced, and the longitudinal mode number dependences of peak power and bandwidth of superposed Brillouin spectrum, and signal to noise ratio, temperature and strain measurement accuracy of the system were analyzed and calculated. The results show that with the increasing of the longitudinal mode number, the signal to noise ratio, temperature and strain measurement accuracy of the system at the end of 25 km long fiber are improved significantly for a multi-longitudinal mode Fabry-Perot laser with a mode interval of 0.141 nm, and the optimal value of temperature and strain measurement accuracy is 3.81 ℃ and 86.69 με respectively, when the longitudinal mode number takes the number of 19.