A method to measure the relative phase distribution of the signal with repetitive pattern was proposed based on ultra-high resolution optical spectrum analysis.After achieved the optical spectrum,the pump with carrier suppression of the Mach-Zehnder modulator was modulated to create a dual-pumping stimulated Brillouin scattering,as dual-channel active filter.By continuously scanning through the wavelength range under test,data acquisition and processing,the relative phase distribution of modulated optical signal was got.The frequency response results show that the dual-channel filter is suitable for phase measuring,and the relative phase distribution in frequency domain of a 5 Gb/s,31 bits optical signal with repetitive pattern modulated using this technique was obtained.