Abstract: In order to study the optical feedback characteristics of the dual frequency laser,a birefringent external cavity was studied throughboth theoretical analysis and experiments.In the experiments,the phenomenon of the fringe frequency doubled was observed.It was found that the resolution of the optical feedback system was doubled and the sensitivity of the optical feedback system was also improved after the modes competition between both the polarized light beams.The work is useful for improving the resolution of the optical feedback systems.