A double diffracting gratings measurement system with the resolution of nanometer order is analyzed. A mathematic model of diffracting Moiré signal and displacement measurement is established. The displacement characteristics of laser Moiré signals are studied by numerical simulation. A precision position device using double diffracting gratings is designed which can achieve precise position measurement and automatic precisionalignment by the differential Moiré signal between two laser Moiré sensors. The differential Moiré positioning technique are used, which greatly improves sensitivity of position signals and positioning precision. The compound control system for precision positioning is set up using the coarse alignment and the fine alignment, which achieves the high alignment accuracy and quick alignment speed. The experimental result shows the alignment device using diffracting gratings can perform positioning accuracy of ±0.5 μm.