The Moire fringes are generated by overlapping of two gratings. Because the Moire fringes reflect the defference of two gratings' frequencies, they have the function of amplification of phase and angles. In this experiment, we use two gratings with the same frequency to generate Moire fringes and use them to measure the long focus length of lens. The theoretical formula is deduced. When a lens is placed between two gratings, its Moire fringes are a group of parallel lines, and the relation of the slope of the Moire fringes with the long focus length of the lens is certainty. We use CCD to get the images of the Moire fringes, and figure out the focus length of the lens with them.