A new technique of mask division for improving the sharpness of mask edge is put forward, which can be used to solve the lowpass filtering problem of the reduction lens in the system of mask fabrication with electrically addressed spatial light modulators. A mask image with high frequency is sampled with fixed or variable low frequency. In each period of the low frequency mask, only one part of the initial mask is included. Using real-time mask technique, multiple division masks are exposed sequentially and the high frequency mask is restored. For fabrication of binary grating, the relative lost efficiency caused by aperture limit of the lens can be decreased from 35.23% to 6.09%. And part of the low frequency energy is moved to the middle and high frequency. The sharpness of the mask edge is ameliorated evidently. Using division mask, complex shape mask or multi-level mask can be changed into binary mask, which is easy to be designed and realized. Moreover, the transition of multi-level gray to binary, that is, black and white, can eliminate the disturbing effect of the refresh rate of screen.