Based on Talbot effect and Moiré fringes of Ronchi gratings,focal length measurement of the long focal length is carried out. The focal length of measured lens can be figured out accurately according to its relation to the slope coefficient of Moiré fringes. To get the slope of Moiré fringes, the image of Moiré fringes is mainly filtered or smoothed by mathematics morphological method to enhance the contrast of fringes effectively. Then the filtered image is processed by thinning to single pixel width. The fringes are labeled to fitting lines to calculate the slope coefficient accurately,and the error of focal length is 0.10%.