The position and period of grating fringes will vary against the substrate during double beam interference exposure due to temperature variation, air flow, vibration and other factors in the fabrication of largesize holographic gratings. It will result in the decrease of grating contrast. The dynamic change of fringes was analyzed during largesize grating exposure. The RootMeanSquare (RMS) errors of fringe translation and period in 3 hours are 1.87 and 1.20 period respectively. The corresponding simulative grating contrasts are 12.83% and 67.37% respectively. A fringe locking system composed of three monitoring subsystems, a computation and control subsystem and two piezo nanopositioners was set up. The translation and period of fringes can be locked at the same time. The groove shape and contrast of gratings are improved significantly after locking. The precision of this fringe locking system: the 3σ value of fringe translation and corresponding contrast are 0.009 period and 99.99% respectively; the RMS error of period change and corresponding contrast are 0.017 period and 99.77% respectively. This locking system satisfies the requirements of fringe stability for the exposure of largesize gratings.