A Brillouin optical time domain reflectometer sensing system is proposed based on external modulation of the multi-longitudinal mode Fabry-Perot laser and frequency-shifted local optical heterodyne detection. The principles of stimulated Brillouin scattering threshold improvement and heterodyne detection of frequency-shifted local light from the same laser and Brillouin scattering from the sensing fiber are analyzed, and the mathematical expression of signal-to-noise ratio of the system is deduced. The dependence of the peak power and bandwidth of superposed Brillouin spectrum, the signal-to-noise ratio and the measurement accuracy of Brillouin frequency shift on the longitudinal mode number, and the relationship between the longitudinal mode number and the optimal measurement accuracy for different pulse widths are theoretically studied. The corresponding fitting formulas are obtained by calculation. The results show that with the increase of longitudinal mode number, the signal-to-noise ratio, temperature and strain measurement accuracies 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 a pulse with a peak power of 100 mW and a width of 50 ns for single-longitudinal mode. Especially, compared with single longitudinal mode, the signal-to-noise ratio increases by 11.73 dB, and the optimal temperature and strain measurement accuracies of 3.2 ℃ and 70.8 με are achieved, respectively, when the longitudinal mode number is 20. The optimal measurement accuracy of Brillouin frequency shift rises rapidly with the increasing pulse width, and the optimal measurement accuracy tends to a constant of 2.9 MHz for pulse width larger than 100 ns.