Highly sensitive interferometer is essential to observation of many physical phenomena. Here a method to realize Ramsey interferometer by using stimulated Brillouin scattering effect is proposed. Two stimulated Brillouin scattering processes are involved in the method. The first one is to prepare the coherence phonons, in which the relative phase is achieved in the free evolution of the phonons, then the Ramsey fringes in the output field can be achieved with the assistance of the second stimulated Brillouin scattering process. Since the method is based on the mean values of the output photons, the observed Ramsey fringes are not sensitive to the noise. In addition, the scheme is feasible with current experimental technology.