This paper studies in-depth the principle of the Vector Brillouin Optical Time Domain Analysis（VBOTDA）system.This system adopts a heterodyne detection scheme on the basis of the conventional BOTDA system

thereby enhancing the system Signal-to-Noise Ratio（SNR）and acquiring Brillouin phase-shift information.Then

it proposes a phase demodulation method

i.e.the spectrum algorithm that combines the superimposed average denoising algorithm with the FFT analysis.Theoretical analysis and experimental demonstration verify its feasibility and effectiveness in that it is effective in demodulating signals with different SNR and for signals with 0.3 Vamplitude and noise standard deviation of 0.5 V

and the phase relative measuring error can be lowered to 0.71% when the spectrum algorithm superimposed 1 000 times is used.