In this paper the effect of the slit filter in a two grating system illuminated by extended light source on the formation of the final Moire fringes is discussed. The phenomena that the final frings will moveand tilt when the slit filter has displacements was analysed. It shows that the tilted fringes are a combination of many segments of fringes. The quantitative relations between the displacements of the slit and the mevement and tilting angle of the fringes are given. All the results are tested experimentally.