TFT-LCD is a very important spatial light modulator in optical information processing. The matrix of TFT -LCD is constructed with a metal mask that covers the inter - pixel spaces, obscuring the thin fihn transistors and preventing light leakage through the unswitched parts of the panel. It result in diffraction. The overlap of adjacent order of diffraction limited the spatial frequency used in optical information processing with TFT - LCD. The analysis shows that the relative amplitude spatial distributings of different diffraction orders are same. This has been checked by the experiment of JTC.