A fringe locking phenomenon in a two beam-laser diode interferometer with strong optical feedback observed in the system is studied in this paper. If a fraction of at least 0.1% of the laser diode output power is returned to it, the fringes are locked even in the presence of the vibrations of an interferometer mirror. The phenomenon is useful for real-time measurement with high accuracy, for it decreasing the measurement error caused by the environment. A theoretical model has been proposed which is based on a four-mirror equivalent Fabry-Porot cavity including the laser cavity and the interferometer. Through theoretical deduction and numerical analysis using the model, the resonant condition of the laser diode was concluded and the dependence of wavelength on the change of the optical path difference of the interferometer calculated, then the fringe locking phenomenon can be well explained. The fringe locking phenomenon is also experimentally studied. It can be observed that when the mirror is driven slowly with a sawtooth wave with a frequency of 100Hz and an amplitude of 1.2μm the fringe locking phenomenon occured. Dependence of the wavelength on the optical path difference is measured too.